MB-12179: Make DcpBackfill class a state machine

Change-Id: I7e14fba6dcd91810ea83f8c5836ce4e0507a926f
Reviewed-on: http://review.couchbase.org/41835
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>