Summary:
Add an implementation of `WorkerProvider` to `IOThreadPoolDeadlockDetectorObserver`.

- On thread start, the observer creates a `TaskStallDetector` and provides it a `ThreadIdCollector` containing the TID of the thread that started
- The thread will be blocked from exiting until any existing `WorkerProvider::KeepAlive`s are destroyed

Differential Revision: D31963855

fbshipit-source-id: cd749d76a433684a8280d95a4dbff441e32bfda9