MB-11721: Send acks for snapshot markers during the takeover phase

By sending acks for snapshot markers we can make sure that all items
hve actually been processed on the new master before sending any
set vbucket state messages. This change will reduce the time that
a vbucket might be unavailable due to state changes.

Change-Id: I70b64035c4684754bfcdb632f29cb83fecf161a1
Reviewed-on: http://review.couchbase.org/39375
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>