PendingAntiCompaction currently cancels all ongoing compactions to be able to grab the sstables it requires - it should;
- avoid stopping ongoing anticompactions - for example, if we have an sstable [0, 100] and are anticompacting it on [0, 50] - now if we start a new incremental repair on [51, 100] we will (try to) stop the first one. Instead we should fail the new incremental repair request.
- avoid stopping unrelated regular compactions - we should only stop regular compactions for the sstables the new anticompaction needs
This requires us to keep track of which sstables are being compacted by a given compaction id.