Major Description
While working on SOLR-7789, I realized it would be easier if I split up the OverseerCollectionProcessor into two parts:
1) General handling of tasks (work queues, etc)
2) Processing a collection handler request
I haven't decided whether the ConfigSet should have its own processor, i.e. OverseerConfigSetProcessor or reuse at least the thread for the OverseerCollectionProcessor, but in either case this refactoring will be helpful. That is, if the ConfigSet processing has its own processing, I can reuse the general handling of tasks part. If the ConfigSet processing reuses the OverseerCollectionProcessing thread, I won't complicate the implementation with ConfigSet operations.