def terminate
      @mtx.synchronize do
        return false if not @running
        Message.new(TERM).send_to(self)
        await_handler_death
        true
      end
    rescue IO::EAGAINWaitWritable
      # same as in #<<
      retry
    end
def terminate
      @mtx.synchronize do
        return false if not @running
        Message.new(TERM).send_to(self)
        await_handler_death
        true
      end
    rescue IO::EAGAINWaitWritable
      # same as in #<<
      retry
    end
Tells the actor to terminate and waits for it. Idempotent.
 @return [Boolean] whether it died just now (+false+ if it was dead
   already)