MB-11827: Check the checkpoint id instead of backfill state

I've noticed that in the past these values haven't always been
kept in sync and that it is much safer to just check the checkpoint
id.

Change-Id: Icf51588d4ec411bc507bea8cff7505b6642e8c0b
Reviewed-on: http://review.couchbase.org/39960
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>