Botan TLS

The #1661 (closed) to #1664 (closed) tickets assume the code się using boost.SSL lib, which uses OpenSSL internally. The goal of this ticket is to try to come up with a solution that uses Botan instead.

If it’s too difficult or requires too many changes compared to boost.ssl, then it is acceptable to have TLS support only when compiling with OpenSSL. If compiling with Botan, we could disable TLS support. Obviously, getting it work would be better...

(done (i.e. it works) and the description obviously needs more edit than this one)