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
MB-9107: Backfill optimization part 1

Change-Id: I51aad158e9bab15cc8fb78f3c140c1d87c224101
MB-9107: Backfill optimization part 2

Change-Id: I4c1c16ff26e32f04e37de4a8dce4d4014f938dd5