MB-100 sligth ebucketmigrator_srv re-factoring

needed to reuse the code in upr replicator

Change-Id: I353cee570e103d60f8a6a499da582cd3489e8655
Reviewed-on: http://review.couchbase.org/29957
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>