def process_queue #:nodoc:
      messages_come = false
      @connections.each do |connection|
        while not connection.msg_queue.empty?
          msg = connection.msg_queue.pop
          messages_come = true
          if @callback
            @callback.call(msg)
          end
        end
      end
      messages_come
    end
def process_queue #:nodoc:
      messages_come = false
      @connections.each do |connection|
        while not connection.msg_queue.empty?
          msg = connection.msg_queue.pop
          messages_come = true
          if @callback
            @callback.call(msg)
          end
        end
      end
      messages_come
    end
execute callback for all queued messages.
 This is called by {Node#spin_once}.
 It checks all queues of connections and callback for all messages.
 @return [Bool] some message has come or not.