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

This reverts commit 2469a551fabdcd68d6e589adde1181d7cf81ded3.

I meant to make things a little bit cleaner with this change. It
wasn't actually meant to fix a particular problem. Since it instead
just created issues we can revert it without having to find any
other fix.

Change-Id: Id67bffea9233b826a32b7907a9284ce80b7ab11a
Reviewed-on: http://review.couchbase.org/38314
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>