MB-100: fix merge_keys for specific case

test load_with_async_ops_and_joint_sets when doc-ops=create-delete,doc-ops-dest=create-update-delete
http://qa.hq.northscale.net/job/ubuntu-64-2.0-biXDCR-all/256/consoleFull

remove from valid keys on src cluster if keys were deleted on dest node

Change-Id: Ia2be972ef42967170bd52b678d6346a066c0893a
Reviewed-on: http://review.couchbase.org/24815
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>