The mirror_super signals the commit state to the control thread via an
mbox, and this mbox is moved to control. It was owned by mirror_super,
but the problem with that is that mirror_super can free the mbox before
the control client has been scheduled to receive the message. If it's
owned by the control object, that can't happen.
At the moment, a first-pass failed migration will retry. This is wrong,
it should abort. However, to make that happen the mirror supervisor
needs to know the commit state of the mirror thread. With a self_pipe
mirror commit signal that information wasn't there.
