Cleans up worker tasks and adds a new feature for task center to get a TaskGuard for unmanaged tasks that should be cancelled on drop.
This also waits for worker shutdown explicitly before shutting down the rest of the system to allow partitions to drain cleanly with potential improvement in a follow-up PR that would let rocksdb flush to happen once PartitionStoreManager is dropped.

Bonus: This includes a minor fix to let PPM cancel processors while they are still starting up, if they are still opening partition store or mid-initialization delay.