Concurrency: `ForkJoinPoolScheduler#schedule(..)` can intermittently lose messages

[SereneAnt]

In ForkJoinPoolScheduler#schedule(..), an actor can intermittently skip messages.

Possible scenarios (others possible): 1.

• The single actr created;
• Thread A adds a runnable to the actr's mailbox;
• Thread A locks the mailbox and runs the runnable;
• Thread B adds a runnable, the actor's mailbox is still locked, so the 'runnable.run()' is skipped.
• Thread A unlocks the mailbox;

2. • The single actr created;
   • Thread A adds a runnable to the actr's mailbox;
   • Thread A locks the malebox and runs the runnable;
   • Thread B adds a runnable to the same mailbox;
   • Thread A removes the last runnable (from thread B) without execution.

The result: The message (runnable) from thread B is lost.

[zakgof]

Fixed in actr 0.1.0