Complete CriticalEvent Logic

[VigneshVSV]

The idea of the CriticalEvent object was that one can receive acknowledgement for the server sent event. There is a similar idea in the thing description to support dataResponse for events.

Possible steps involved

• solidify the CriticalEvent concept based on the suggestion given by the Thing Description spec. This may even involve renaming CriticalEvent to something else to indicate any response apart from an acknowledgement is also supported.
• Support dataResponse in the init of CriticalEvent
• Create a message in ZMQ & modify EventLoop.execute_once which will call the CriticalEvent's data response accepting method
• Define how errors will be raised to the user
• Idea should work for both HTTP & ZMQ transport