Merge: libevent: support UNIX domain sockets and add a test and an example

Libevent server can now listen on UNIX domain sockets by calling `ConnectionFactory::bind_unix`. The method to listen on TCP ports was renamed to `bind_tcp` for clarity.

This PR intro two new supporting programs in the form of a parallel test for a libevent server and a minimal usage example. If we already had these let me know, because otherwise these two were long overdue.

This PR is built on the work of @matthmsl in #2708, thank you!

Close #2679

Pull-Request: #2726