Separate out starting a sink into dedicated functions.

Change-Id: I850f2393102c20429504e3854556be11a5d27b9b
Reviewed-on: http://review.couchbase.org/33019
Reviewed-by: Artem Stemkovski <artem@couchbase.com>
Tested-by: Artem Stemkovski <artem@couchbase.com>