Lack of compaction tooling for LeveledCompactionStrategy

This is a highly frustration-driven ticket. Apologize for roughness in tone

Background: I happen to have a partition key with lots of tombstones. Sadly, I happen to run LeveledCompactionStrategy (LCS). Yes, it's probably my mistake to have put them there but running into tombstone issues seem to be common for Cassandra, so I don't think this ticket can be discarded as simply user error. In fact, I believe this could happen to the best of us. And when it does, there should be a quick way of correcting this.

Problem: How does one handle this? Well, for DTCS one could issue a compaction using `nodetool compact`, or one could use the forceUserDefinedCompaction MBean. Neither of these work for LCS (shall I also say DTCS?).

Workaround: The only options AFAIK are

1. to lower "gc_grace_seconds" and "wait it out" until the Cassandra node(s) has garbage collected the sstables. This can take days.
2. possibly lower `tombstone_threshold` to something tiny, optionally lowering `tombstone_compaction_interval ` (for recent deletes). This has the implication that nodes might start garbage collecting a ton of unrelated stuff.
3. variations of "delete some or all your sstables" and run a full repair. Takes ages.

Proposed solution: Either

• Make forceUserDefinedCompaction support LCS, or create a similar endpoint that does something similar.
• make something similar to `nodetool compact` work with LCS.

Additional comments: I read somewhere where someone proposed making LCS default compaction strategy. Before this ticket is fixed, I don't see that as an option.

Let me know what you think (or close if not relevant).