Implement general controller stats/counters

Hi devs,

As discussed on mail, we should have basic controller stats/counters available through REST APIs.

I am thinking for example:

• REST packets sent/received by controller, if possible separated by type like GET, POST, etc...
• Error counters like REST requests not being processed for any reason
• OF packets sent/received by controller, if possible separated by message type like PACKET_IN
• Error counters like OF packets not being processed for any reason
• Total number of flows maintained in the controller
• Performance monitoring like CPU, RAM usage, etc...

Thanks/Luis