handler: add option to use new event loop

[jkarneges]

This adds support for the new event loop in the handler, controlled by a config option (new_event_loop=true) to make it easy to test it without fully committing to it yet.

I've done some basic testing of common flows (start/stop, accept sessions, publish messages) and things look ok so far. The main thing missing is support for endpoints that use SimpleHttpServer (used by the internal control port and prometheus), as it depends on QTcpSocket which we don't have a substitute for yet. So the config options for those endpoints (push_in_http_port and prometheus_port) need to be omitted when testing, and publishing needs to go over zmq.