apparentlyFirstQueuedIsExclusive

Returns true if the apparent first queued thread, if one exists, is waiting in exclusive mode. If this method returns true, and the current thread is attempting to acquire in shared mode (that is, this method is invoked from .tryAcquireShared) then it is guaranteed that the current thread is not the first queued thread. Used only as a heuristic in ReentrantReadWriteLock.