Modify HA hook to instantiate CmdHttpListener (and enable client MT)

This is the sixth ticket in the HA+MT effort. It covers the modifications of the HA hook to instantiate the CmdHttpListener to be able to support multiple parallel incoming connections, as described in the HA+MT design.

Once this ticket is complete, the server-side of the HA hook with become operational. However, to take advantage of it, #1735 (closed) will be required.