MB-100: skip unchanged vb map chagne msg

Today for some reasons unknown, bucket replicator will receive
"buckets" msg even without vbucket map changes. Althogh it has
no effect to onging replication but it unnecessarily dumps the
unchanged active vbs as "vbucket map chagned", making misleading
traces in logs. This commit would ignore such msg.

Change-Id: I0c4fed95164bfd6e44026e2bb73eb23d43543322
Reviewed-on: http://review.couchbase.org/30976
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>