MB-10400: use better api for opening src master vbucket

Change-Id: I93ce7bab82c53b56cbc4ea5c0606a9c2fe370b60
Reviewed-on: http://review.couchbase.org/34810
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-10400: removed duplication in queue_fetch_loop

Change-Id: I930e163f8d5b39d4f1e327b22ead24425cdb036b
Reviewed-on: http://review.couchbase.org/34811
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-10400: dropped use tgt_master_db

Because it's not used

Change-Id: If8dea776326765bf24c37964566668808b97cd25
Reviewed-on: http://review.couchbase.org/34812
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>