Implements a new REST interface using HTTP/2, for lower latency and multiplexing of multiple requests on a single connection.
Instead of multithreading + blocking I/O, where each (long-poll) client occupies a thread, using event-driven non-blocking I/O, and poll() the HTTP-related sockets (server socket and client connections) together with the ZMQ sockets. For that, integrate the server more closely with the Broker.

The HTTP/3 implementation has been tested manually using Chrome and curl (see README) but is not covered by unit tests yet, as the native HTTP/3 client implementation would be considerable effort (and probably some more kloc's of code...). As agreed upon with @RalphSteinhagen , the native client might be done later possibly with other libraries like libcurl/CPR that require less code than ngtcp2, given that the client side is not the performance-critical part.