Make compaction gain estimate more accurate

Currently the compaction gain estimation process only takes the current head into account when calculating the retained size. We could make it more accurate by also taking in memory references into account. This would prevent compaction from running when many in memory references would later on prevent segments from being cleaned up.

Also for OAK-2862, we would need a way to include the segments used for the persisted compaction map in the retained size.

While at it, we could try to improve logging so information on how much space is retained by the current head, in memory references and the persisted compaction map would be logged separately.