HA+MT:Create config::CmdHttpListener
This is the first ticket in the HA+MT effort. It covers the implementation of the CmdHttpListener class as described in the HA+MT design:
The class will be added to the libkea-cfgclient library (i.e isc::config namespace).
At the completion of the ticket, the class should be fully functional and unit tested, but not instantiated or referenced in production runtime code.