Use Barrier Message to ensure switch finish processing all previously-received messages

If you want to get the barrier reply , some steps need to be done:
- Look at line 1225 from
  floodlight / src / main / java / net / floodlightcontroller / core / internal / OFChannelHandler.java 
- add  "h.dispatchMessage(m);" in this function , then the function will become:
   void processOFBarrierReply(OFChannelHandler h, OFBarrierReply m) throws IOException {
                      h.dispatchMessage(m);
    }