Request / Response: Formalize the request/response pattern within MQTT

[andsel]

Formalize the request/response pattern within MQTT and provide the Response Topic and Correlation Data properties to allow response messages to be routed back to the publisher of a request. Also, add the ability for the Client to get configuration information from the Server about how to construct the response topics.

Implementation

Implement the management of response information header in CONNACK. When a client send a CONNECT with request response information property set, the server reserve a subtree of the subscription topic so that client can receive response publishes for request-response flow.
The server reserve a subtree specific for the connecting (requester) client, for example /reqresp/reponse/<client id> and configure read access for the client, and write access for the rest (*). This is needed to automatically provide a response topic configured with correct access rights.