MB-12751: Return KEY EEXISTS for locked objects

Change-Id: I5eb8a24337a81fed861cc8140bbdcf1a132acd90
Reviewed-on: http://review.couchbase.org/43625
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>