Introduce SlotManager plugin for fine-grained resource management.
- Request slot from TaskExecutor with the actual resource profiles.
- Use ResourceTracker to bookkeep the resource requirements
- Introduce TaskExecutorTracker, which bookkeep the total / available resource and slot assignment for registered task executor.
- Bookkeep task manager total and available resources.
- Bookkeep slot allocations and assignments.
- Intorduce PendingTaskManager.
- Map registered task executors to matching PendingTaskManager.