MB-8453 Maintain the TAP connection list for each vbucket

When we receive a mutation event for a given vbucket, we
should check if any of its replicator TAP connections is
in a paused state, and notify that event to the paused
TAP connections, so that they can immediately send it
to their replicas.

To address the above issue, this change maintains a
separate list of TAP connections for each vbucket.
The list gets updated when an TAP connection is closed or
an TAP connection's vbucket list is changed.

Change-Id: I5ca49fe17fbeb406da3ebcc046285ddc164735eb
Reviewed-on: http://review.couchbase.org/27972
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>