clean up inconsistencies around deletes

Compaction clears all cells behind any found 'delete' cell. This makes for an inconsistency in that before the compaction runs, you can do a get or scan on a timestamp older than the delete cell and get results. Confusing.

Either preserve all cells for all time (or until > MAX_VERSIONS or TTL) or else make it so presence of a delete cell in the future stops client fetching anything older (expensive).