crimson/os/seastore: add space accounting to segment_cleaner and wire in

crimson/os/seastore: add space accounting to segment_cleaner and wire in

Adds support for space accounting to SegmentCleaner and wires into
Journal, Cache, and tests.

SegmentCleaner has two tracking implementations, SpaceTrackerSimple
and SpaceTrackerDetailed.  SpaceTrackerSimple simply keeps a count
of live bytes and is intended to be the normal implementation.
SpaceTrackerDetailed maintains a bitmap and is simply useful
for debugging unit tests.  It may be removed in the future.

Signed-off-by: Samuel Just <sjust@redhat.com>