Range tombstones/delete_range/delete_prefix

[marvin-j97]

Considerations

• Need to check tombstone deletion range when doing point reads and range reads
• Need to fragment tombstones so in Levelled a segment only has the range tombstones of its range (important going from L0 to L1)
• Need to correctly evict data on merge: this should happen implicitly as a BoxedIterator should just get the DeleteRangeTable, handle deleted items internally, and the MergeIterator should work implicitly
• If a range delete covers an entire segment or the requested range/prefix, any range/prefix scan over it should short-circuit to immediately return None
• Segments need to access tree-level DeleteRange memtable and sealed ones, too