Sequence numbers are assigned through the CheckpointManager

Change-Id: If3fdf08e1091ad36ea8bcd30b10cd3803db009ae
Add the by sequence number to the tap engine specific section

Change-Id: I501bd9b044e929bda324dd8cb2c2370c9900029f
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