Move the mirror commit state mbox to struct control

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.

src/mbox.c

@@ -55,11 +55,11 @@ void * mbox_receive( struct mbox * mbox )
 		mbox->full = 0;
 		result = mbox->contents;
 		mbox->contents = NULL;
+
 		while( 0 != pthread_cond_signal( &mbox->emptied_cond));
 	}
 	pthread_mutex_unlock( &mbox->mutex );
 
-
 	return result;
 }