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

[iAnt]

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

A possible scenario (one of ?):

• 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 to the same mailbox;
• Thread A removes the last runnable from thread B (which is lost !!!);
• The mailbox is still not empty, so the first runnable is executed again;

[Oleksandr Zakusylo]

Fixed in actr 0.1.0