[New Scheduler] Refactor and unit test queuing logic

[wuisawesome]

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

[wuisawesome]

Reopening issue to track test coverage