ep-engine: race condition in TaskQueue shutdown

There is a bug in the use of ExecutorThread.state when sleeping a TaskQueue - TaskQueue::_doSleep() doesn't atomically transition the state from RUNNING -> SLEEPING. This can cause a deadlock when shutting down a ExecutorThread:

        Thread A:                           Thread B:

        --------------------------------    ------------------------------

        if (t.state == RUNNING) {  // true

                                            t.state = SHUTDOWN

            t.state = SLEEPING              cb_join_thread(Thread A)

                                            // wait forever

        ...

        if (t.state == SHUTDOWN) { // FALSE

          exit(0) // NEVER REACHED

        }

Fix by changing ExecutorThread.state to be an AtomicValue, and use compare-and-exchange to move from RUNNING -> SLEEPING (and SLEEPING -> RUNNING).

Note: This was previously fixed in 4.0.0 prior to release - see http://review.couchbase.org/43134 Backporting as 3.x ThreadSanitizer runs have flagged it as an issue there also.