[VOL-1913] Replies with errors of the form "transaction-not-acquired" are now ignored.

Also had to add a conditional branch in case no valid responses are received (the last thread to exit must clean up the connection).
The rw cores will now respond with a "transaction-not-acquired" error in place of "failed-to-seize-request" or COMPLETED_BY_OTHER.

Change-Id: I199d4a0091ba4fc1db5b8097adbad951408e5034
diff --git a/rw_core/core/transaction.go b/rw_core/core/transaction.go
index 0d30340..b2d01a4 100644
--- a/rw_core/core/transaction.go
+++ b/rw_core/core/transaction.go
@@ -37,6 +37,8 @@
 import (
 	"github.com/opencord/voltha-go/common/log"
 	"github.com/opencord/voltha-go/db/kvstore"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
 	"time"
 )
 
@@ -50,6 +52,8 @@
 	STOPPED_WAITING_FOR_KEY
 )
 
+var errorTransactionNotAcquired = status.Error(codes.Canceled, "transaction-not-acquired")
+
 const (
 	TRANSACTION_COMPLETE = "TRANSACTION-COMPLETE"
 )