compactions: determine eventual output level

In a compaction, the files we create can move down via move compactions. We can check how many levels under the output levels are empty within the compaction bounds to determine the likely "eventual" output level. We should use that level for target file size and compression settings.

For any given file, the eventual output level might still be an underestimate because individual output files could move down more. Still, this should be an important improvement in many common cases (like writing in a new key space).

Jira issue: PEBBLE-496