diff --git a/vendor/golang.org/x/sync/AUTHORS b/vendor/golang.org/x/sync/AUTHORS
new file mode 100644
index 0000000..15167cd
--- /dev/null
+++ b/vendor/golang.org/x/sync/AUTHORS
@@ -0,0 +1,3 @@
+# This source code refers to The Go Authors for copyright purposes.
+# The master list of authors is in the main Go distribution,
+# visible at http://tip.golang.org/AUTHORS.
diff --git a/vendor/golang.org/x/sync/CONTRIBUTORS b/vendor/golang.org/x/sync/CONTRIBUTORS
new file mode 100644
index 0000000..1c4577e
--- /dev/null
+++ b/vendor/golang.org/x/sync/CONTRIBUTORS
@@ -0,0 +1,3 @@
+# This source code was written by the Go contributors.
+# The master list of contributors is in the main Go distribution,
+# visible at http://tip.golang.org/CONTRIBUTORS.
diff --git a/vendor/golang.org/x/sync/LICENSE b/vendor/golang.org/x/sync/LICENSE
new file mode 100644
index 0000000..6a66aea
--- /dev/null
+++ b/vendor/golang.org/x/sync/LICENSE
@@ -0,0 +1,27 @@
+Copyright (c) 2009 The Go Authors. All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+   * Redistributions of source code must retain the above copyright
+notice, this list of conditions and the following disclaimer.
+   * Redistributions in binary form must reproduce the above
+copyright notice, this list of conditions and the following disclaimer
+in the documentation and/or other materials provided with the
+distribution.
+   * Neither the name of Google Inc. nor the names of its
+contributors may be used to endorse or promote products derived from
+this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/vendor/golang.org/x/sync/PATENTS b/vendor/golang.org/x/sync/PATENTS
new file mode 100644
index 0000000..7330990
--- /dev/null
+++ b/vendor/golang.org/x/sync/PATENTS
@@ -0,0 +1,22 @@
+Additional IP Rights Grant (Patents)
+
+"This implementation" means the copyrightable works distributed by
+Google as part of the Go project.
+
+Google hereby grants to You a perpetual, worldwide, non-exclusive,
+no-charge, royalty-free, irrevocable (except as stated in this section)
+patent license to make, have made, use, offer to sell, sell, import,
+transfer and otherwise run, modify and propagate the contents of this
+implementation of Go, where such license applies only to those patent
+claims, both currently owned or controlled by Google and acquired in
+the future, licensable by Google that are necessarily infringed by this
+implementation of Go.  This grant does not include claims that would be
+infringed only as a consequence of further modification of this
+implementation.  If you or your agent or exclusive licensee institute or
+order or agree to the institution of patent litigation against any
+entity (including a cross-claim or counterclaim in a lawsuit) alleging
+that this implementation of Go or any code incorporated within this
+implementation of Go constitutes direct or contributory patent
+infringement, or inducement of patent infringement, then any patent
+rights granted to you under this License for this implementation of Go
+shall terminate as of the date such litigation is filed.
diff --git a/vendor/golang.org/x/sync/semaphore/semaphore.go b/vendor/golang.org/x/sync/semaphore/semaphore.go
new file mode 100644
index 0000000..ac53e73
--- /dev/null
+++ b/vendor/golang.org/x/sync/semaphore/semaphore.go
@@ -0,0 +1,127 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package semaphore provides a weighted semaphore implementation.
+package semaphore // import "golang.org/x/sync/semaphore"
+
+import (
+	"container/list"
+	"context"
+	"sync"
+)
+
+type waiter struct {
+	n     int64
+	ready chan<- struct{} // Closed when semaphore acquired.
+}
+
+// NewWeighted creates a new weighted semaphore with the given
+// maximum combined weight for concurrent access.
+func NewWeighted(n int64) *Weighted {
+	w := &Weighted{size: n}
+	return w
+}
+
+// Weighted provides a way to bound concurrent access to a resource.
+// The callers can request access with a given weight.
+type Weighted struct {
+	size    int64
+	cur     int64
+	mu      sync.Mutex
+	waiters list.List
+}
+
+// Acquire acquires the semaphore with a weight of n, blocking until resources
+// are available or ctx is done. On success, returns nil. On failure, returns
+// ctx.Err() and leaves the semaphore unchanged.
+//
+// If ctx is already done, Acquire may still succeed without blocking.
+func (s *Weighted) Acquire(ctx context.Context, n int64) error {
+	s.mu.Lock()
+	if s.size-s.cur >= n && s.waiters.Len() == 0 {
+		s.cur += n
+		s.mu.Unlock()
+		return nil
+	}
+
+	if n > s.size {
+		// Don't make other Acquire calls block on one that's doomed to fail.
+		s.mu.Unlock()
+		<-ctx.Done()
+		return ctx.Err()
+	}
+
+	ready := make(chan struct{})
+	w := waiter{n: n, ready: ready}
+	elem := s.waiters.PushBack(w)
+	s.mu.Unlock()
+
+	select {
+	case <-ctx.Done():
+		err := ctx.Err()
+		s.mu.Lock()
+		select {
+		case <-ready:
+			// Acquired the semaphore after we were canceled.  Rather than trying to
+			// fix up the queue, just pretend we didn't notice the cancelation.
+			err = nil
+		default:
+			s.waiters.Remove(elem)
+		}
+		s.mu.Unlock()
+		return err
+
+	case <-ready:
+		return nil
+	}
+}
+
+// TryAcquire acquires the semaphore with a weight of n without blocking.
+// On success, returns true. On failure, returns false and leaves the semaphore unchanged.
+func (s *Weighted) TryAcquire(n int64) bool {
+	s.mu.Lock()
+	success := s.size-s.cur >= n && s.waiters.Len() == 0
+	if success {
+		s.cur += n
+	}
+	s.mu.Unlock()
+	return success
+}
+
+// Release releases the semaphore with a weight of n.
+func (s *Weighted) Release(n int64) {
+	s.mu.Lock()
+	s.cur -= n
+	if s.cur < 0 {
+		s.mu.Unlock()
+		panic("semaphore: bad release")
+	}
+	for {
+		next := s.waiters.Front()
+		if next == nil {
+			break // No more waiters blocked.
+		}
+
+		w := next.Value.(waiter)
+		if s.size-s.cur < w.n {
+			// Not enough tokens for the next waiter.  We could keep going (to try to
+			// find a waiter with a smaller request), but under load that could cause
+			// starvation for large requests; instead, we leave all remaining waiters
+			// blocked.
+			//
+			// Consider a semaphore used as a read-write lock, with N tokens, N
+			// readers, and one writer.  Each reader can Acquire(1) to obtain a read
+			// lock.  The writer can Acquire(N) to obtain a write lock, excluding all
+			// of the readers.  If we allow the readers to jump ahead in the queue,
+			// the writer will starve — there is always one token available for every
+			// reader.
+			break
+		}
+
+		s.cur += w.n
+		s.waiters.Remove(next)
+		close(w.ready)
+	}
+	s.mu.Unlock()
+}