MB-11192: Waking up backfills when memUsage falls below HWM

(HWM: HighWaterMark)

Wake up backfillManagerTasks when memory is freed in the
following scenarios:
    - ItemPagerTask evicts items
    - CheckpointRemoverTask removes closed
      unreferenced checkpoints

Change-Id: Ie1e773b74389104e709ac0743328f31b6d3aba02
Reviewed-on: http://review.couchbase.org/43196
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>