Allow multiple overlapping sstables in L1

Currently, when a normal L0 compaction takes place (not STCS), we take up to MAX_COMPACTING_L0 L0 sstables and all of the overlapping L1 sstables and compact them together. If we didn't have to deal with the overlapping L1 tables, we could compact a higher number of L0 sstables together into a set of non-overlapping L1 sstables.

This could be done by delaying the invariant that L1 has no overlapping sstables. Going from L1 to L2, we would be compacting fewer sstables together which overlap.

When reading, we will not have the same one sstable per level (except L0) guarantee, but this can be bounded (once we have too many sets of sstables, either compact them back into the same level, or compact them up to the next level).

This could be generalized to allow any level to be the maximum for this overlapping strategy.