ep-engine windows unit tests hit memory corruption ExecutorPool

As reported by Trond on windows, make test failed on windows with a memory corruption in priority_queue
This happens only during shutdown of ep-engine
------------ More details from windows -----------------------------
Crash happens during shutdown, the thread states are as follows...
Main thread: ~EventualyPersistentStore() -> stopFlusher() trying to wake Flusher Shard 1 Task and getting blocked on mutex to writer queue
Snapshot Task Thread: finished VBSnapshot task and trying to addLogEntry blocked on writer queue mutex
Flusher Shard 1 Task Thread: completeFlush() trying to schedule a VBSnapshot Task blocked on writer queue mutex
Crashing Thread: Done with Reader Queue and now trying to move the flusher task from the future queue to ready queue.
The corrupted queue has 3 Flusher Shard Tasks that are sleeping with infinite sleep time and 1 StatSnapshot task set to wake up soon.