[New Scheduler] Refactor and unit test queuing logic

## Tracking the refactoring and testing of the new scheduler 

## Progress

#9491 provides a starting point 

## TODO
* Need unit test coverage of `ClusterTaskManager` 
* Remove `ClusterTaskManager::DispatchScheduledTasksToWorkers`'s dependency on `WorkerPool` or find a convenient way to mock `WorkerPool` 


