MB-9103: Update checkpoint stats to include persisted cursor

Change-Id: Idbba533289ac81187c8c1d9b9b2c4912986c71d6
MB-9103: Insert tap cursor in checkpoint containing persisted cursor

Change-Id: I1869c7b33c70473b0ec97e18b4fd9b474de07227
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