[BP] MB-10406 Unlock a locked item on replica vbucket for replication

GetAndLock causes the replication or rebalance operation to fail
in the following scenario:

1) Rebalance starts and is in progress
2) Active vbucket 0 is moving from Node 1 to Node 2
3) Item A on active vbucket 0 on Node 1 is locked through
   getLocked() API while vbucket 0 is still being transferring.
4) Active vbucket 0 is transferred to Node 2. Node 1 changes
   vbucket 0 state to replica.
5) Item A on active vbucket 0 on Node 2 is updated through
   Set() API.
6) Node 2 tries to replicate the updated Item A to the node 1,
   but receives an error code (ENGINE_KEY_EEXISTS) from Node 1
   because Item A is still locked due to the step 3).
7) Node 2 closes the TAP connection to Node 1 because of the
   above error code, which is a bug in the engine side.

To resolve the above issue, we should unlock a locked item on
replica vbucket.

Change-Id: I5300b9f8f1864ed8ff5388dc7a621c205c589047
Reviewed-on: http://review.couchbase.org/43146
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>