[VOL-1862] rwCore waits for kafka and KV Store

This commit is a cherry pick into the master branch from the
voltha 2.1 branch of patch https://gerrit.opencord.org/#/c/15030/

Change-Id: I8a306c8b37ad700ef8234466919e0604e14787cd
diff --git a/db/kvstore/client.go b/db/kvstore/client.go
index 937eefe..67c9219 100644
--- a/db/kvstore/client.go
+++ b/db/kvstore/client.go
@@ -77,7 +77,7 @@
 	return evnt
 }
 
-// Client represents the set of APIs  a KV Client must implement
+// Client represents the set of APIs a KV Client must implement
 type Client interface {
 	List(key string, timeout int, lock ...bool) (map[string]*KVPair, error)
 	Get(key string, timeout int, lock ...bool) (*KVPair, error)
@@ -90,6 +90,7 @@
 	Watch(key string) chan *Event
 	AcquireLock(lockName string, timeout int) error
 	ReleaseLock(lockName string) error
+	IsConnectionUp(timeout int) bool // timeout in second
 	CloseWatch(key string, ch chan *Event)
 	Close()
 }