MB-100: ignore irrelevant src_db_updated msg

Today for some reason unknown, XDCR will receive src_db_updated
msg for a vbucket the bucket replicator does not care. For example
in a 2-node cluster, one node responsible for replicating all
vb 0 through vb 511, but it also receives source db update msg
for vb 512 through vb 1023, which spam the logs with useless
information and make valid trace difficult to find.

Change-Id: I42db9fc60ef72763ceb9b259c27774ac1d28fab5
Reviewed-on: http://review.couchbase.org/30975
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>