DISCARD_OLDEST as rejection policy must discard messages when max buffer size is 0

DISCARD_OLDEST as rejection policy must discard messages when max buffer size is 0.

If the max buffer is more than 0 and maxThreadsActive > 1, it must work as now:

discarding the oldest request in the workQueue and attempting again to execute de runnable. This won't generate a stackoverflow as in the worst case, the it will be enqueue in the workqueue (buffer).