MB-12751: Return KEY EEXISTS for locked objects

Change-Id: I55a1a4e7ff71f222a268650d56d01da58c172b6a
Reviewed-on: http://review.couchbase.org/43572
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>