Dynamic budget for DRS compactions

Clusters have busier and quieter periods, so by default Kudu leverages the latter to schedule compactions because during the former it's mostly flushing.

A further improvement would be to somehow recognize that a tserver is mostly scheduling DRS compactions and to start giving them bigger and bigger budgets. Compacting more DRSes at a time lowers the overall write amplification, by running the risk of compacting for too long and not be able to schedule important flushes. We could lower the risk by re-adding an emergency flush thread, and/or making it possible to cancel tasks.