MB-12179: Move dcp backfill into its own file

We are going to be making a fair amount of changes to this class
and it constitutes having its own file.

Change-Id: I4139ef360fdb823b16348c2c1bc010868f47c5d8
Reviewed-on: http://review.couchbase.org/41834
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>