MB-11725: Add an assert to make sure that the disk seqno <= mem seqno

This assert will cause a crash if we try to set the disk seqno
greater than the memory seqno. It will allow use to better debug the
issue where backfilling doesn't read any items. It is also a condition
that if not handled properly can result in data loss.

Change-Id: Ic5935649af4a837a21188013fe02a4dde2b7fb61
Reviewed-on: http://review.couchbase.org/39959
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>