Elides the need to allocate an AkkaForkJoinTask for Mailbox-submission to...

... registerForExecution

by having Mailbox extend ForkJoinTask and using the fact that `ForkJoinTask.exec` when returning
`false` does not set completion on the task, so it is free to be resubmitted to the `ForkJoinPool`
without reinitialization.
Yes, as per the specification of the `execute` contract: http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/Executor.html#execute(java.lang.Runnable)