This splits compactions that are to produce more than one
output sstable into tasks that can execute in parallel.
Such tasks share a transaction and have combined progress
and observer. Because we cannot mark parts of an sstable
as unneeded, the transaction is only applied when all
tasks have succeeded. This also means that early open
is not supported for such tasks.

At this time the new parallelization mechanism is not taken
into account by the thread allocation scheme, and thus
some levels may take more resources than they should.
Because of this limitation (which should be fixed in the
near future), the new behaviour is off by default.

Also:
- Adds a flag to combine non-overlapping sets in major
  compactions to reshard data, as major compactions can
  can now be executed as a parallelized operation.

- Changes SSTable expiration to be done in a separate
  getNextBackgroundCompactions round to improve the
  efficiency of expiration (separate task can run quickly
  and remove the relevant sstables without waiting for
  a compaction to end).

- Applies small-partition-count correction in
  ShardManager.calculateCombinedDensity.

…put_shards