MB-11325: Use isBackfillPhase instead of checking chk id num

When a checkpoint is in backill phase the checkpoint id is set to
0. We should instead make sure that the vbucket is actually in
backfill phase instead of just checking one of the symptoms of
being in backfill phase.

Change-Id: Ib5be4c18b9d551f7f8ce9abaa1071375ffff293e
Reviewed-on: http://review.couchbase.org/38241
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>