[Epic] Incremental repair

[denesb]

The idea of incremental repair is tied closely with tablets. The vision is that with tablets and the infrastructure to automatically schedule and execute repairs:

• repair will be cheap and quick
• repair will be scheduled automatically
• repair will happen often

Tablets have a nominal size of 5GB, so repairing a single tablet should be reasonably quick. To make repairing a tablet even quicker, we can implement the incremental repair that C* implemented years ago. The idea is that at any given repair, we repair only the data which was written after the last repair. This way, the more often we repair, the cheaper and quicker it is.

Repairing often will allow us to garbage-collect tombstones more often and more aggressively. We can make changes to compaction strategies to make use of this fact.