Fix race condition in mailbox destruction by making mailbox_t destructor block until all callbacks have finished. Mailbox callbacks now take an interruptor signal that is pulsed when the mailbox_t is destroyed.
Catch interrupted_exc_t thrown by mailbox callbacks.
Add clarifying comment.
Catch exception by const reference
Merge branch 'tim_mailbox_destructor_race' into next
Merge branch 'next' into sam_2134_iii