Use standard SHA256 fingerprints

[Jakuje]

Currently, the code in _load_server_publickey() in session.pyx makes SHA1 + hex fingerprints. The OpenSSH normally uses SHA256 + base64 these days. They are ugly and longer, but they are default in OpenSSH so needed when one needs to compare them with output from the ssh-keygen.

It should not be breaking change as the fingerprint should be just the user facing information and not a API.

          We should probably move this FIXME into an issue on the tracker.

Originally posted by @webknjaz in #597 (comment)