MB-9103: Backfills are only from disk

This removes the memory backfill that was used when more than 90%
of the items are resident in memory. We cannot use that mechanism
anymore because we need to ensure that items are sent in order of
increasing sequence number.

Change-Id: I6d1e1994ce558d3cba45693d82e7d51c280b0704
Reviewed-on: http://review.couchbase.org/29114
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>