Remove non empty levels after DB shrinks

If most KVs in a DB are deleted, ideally, the LSM-tree should be adjusted to an ideal state. We know that it will naturally happen in Universal Compaction, and is unlikely to happen in non-dynamic leveled compaction without a full compaction. With some special treatment in dynamic leveled compaction, we should make it possible. If we identify a base level becomes obviously excessive, we should pause L0 -> base level compaction temporarily and compact this level to the next level.

The question was raised by @mdcallag .