MB-12751: Replace for locked key should return EEXISTS

Change-Id: I34184b79f1f94ea6eee39bae26520c126ff87e2a
Reviewed-on: http://review.couchbase.org/43573
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: buildbot <build@couchbase.com>