Receive responses via subscribers

[alexellis]

Receive responses via subscribers

• change enables HTTP body, HTTP status, HTTP headers and any
  error whilst invoking to be received by 1-many subscribers.
• "print response" behaviour is also moved into a subscriber

Tested with the "tester" example connector.

Example:

// ResponseReceiver enables connector to receive results from the
// function invocation
type ResponseReceiver struct {
}

// Response is triggered by the controller when a message is
// received from the function invocation
func (ResponseReceiver) Response(res types.InvokerResponse) {
	if res.Error != nil {
		log.Printf("tester got error: %s", res.Error.Error())
	} else {
		log.Printf("tester got result: [%d] %s => %s (%d) bytes", res.Status, res.Topic, res.Function, len(*res.Body))
	}
}

	receiver := ResponseReceiver{}
	controller.Subscribe(&receiver)

Closes: #4

Signed-off-by: Alex Ellis (VMware) [email protected]

[alexellis]

This may have breaking changes for the other connectors, I will do a release and raise some issues for the connectors we are tracking.

The tests from @Templum would also be good to bring across.

[Templum]

@alexellis I will port them. Should I reference this pr ?