MB-11786: Always create a new checkpoint after backfill phase

Even if we don't get a chk flag in the snapshot marker we still
need to create a new checkpoint if we were previously in backfill
phase.

Change-Id: I147273d23950afd8fdfea98346760075862f39d9
Reviewed-on: http://review.couchbase.org/39896
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>