MB-9663: add to bucket active vb table when starting vb replicator

Today when we start new vb replicator, we do not add it immediately
to the active vb table in bucket replicator, instead, after the vb
replicator finishes initialization, it would report status to bucket
replicator and an entry will be created in the active vb replicator
table within bucket replicator.

This small init time lag may cause issue in corner cases, e.g., when
a vbucket just has one mutation, we may lose this single notification
from couchdb (src_db_updated) because the corresponding vb replicator
has not finished initialization and bucket replicator cannot find its
Pid to relay the notification msg. As a result, this vb replicator
may not be wakened for a long time until next mutation to this vb
arrives. We also see msgs like "get src_db_udpated from vb 12, but
the vb replicator has not initialized yet" in logs.

This fix will add new vb replicator immediately to active vb rep
table in bucket replicator with an empty status entry (but including
the Pid of the new vb replicator). Then all subsequent src_db_updated
msg from couchdb will be captured and relayed to corresponding vb
replicator.

Change-Id: Iba2251065cd3def37d2bf7b3017d1767a23238f9
Reviewed-on: http://review.couchbase.org/30893
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>