Extend compaction interfaces to provide split points at operation start

The current compaction interfaces allow a compaction strategy to split at arbitrary points while it is writing output. In some cases (e.g. UCS) we know in advance where we want to split. Giving this information before the operation starts allows it to operate on multiple segments of the output in parallel, i.e. parallelize within an operation rather than between operations, which can reduce individual operations' duration and significantly improve the DB's chances of keeping up with load, especially on L0.