Refactor CompactionStrategyManager

CompactionStrategyManager grew a decent amount of duplicated code as part of CASSANDRA-9143, which added pendingRepairs alongside the repaired and unrepaired buckets. At this point, the logic that routes sstables between the different buckets, and the different partition range divisions has gotten a little complex, and editing it is tedious and error prone. With transient replication requiring yet another bucket for, this seems like a good time to split some of the functionality of CSM into other classes, and make sstable routing a bit more generalized.