Use ExecutionObserver in EventBase to monitor function not executed EventBaseAtomicNotificationQueueby in EventBase

Summary:
Our service schedules tasks (eSemiFuture/coroutine) on SerialExecutor on top of IOThreadPoolExecutor. When using EventBaseWatchdog to locate long running tasks, I found some long running tasks were not observed by PerThreadExecController. After some investigation, I found there are 2 cases in EventBase that tasks are not executed by handlerReady in EventBaseAtomicNotificationQueue.

1. Short-circuit scenario in runInEventBaseThread which bypasses EventBaseAtomicNotificationQueue.
2. If notification Queue is marked 'internal', some events may be ran "manually" in loopBody.

As a result, the observer logic in EventHandler (EventBaseAtomicNotificationQueue derives from EventHandler) is passed.

Reviewed By: andriigrynenko

Differential Revision: D26563459

fbshipit-source-id: e2741cff373c89a5cd438904f910b23a64fad653