Change rustls_server_connection_get_server_name to return rustls_str

[jsha]

This function was written before we started using rustls_str, and it uses the pattern of writing to a caller-provided buffer. That pattern is quite awkward to use, and rustls_str fits better, so we should use that instead.

Note: #178 (lifetime issues for rustls_connection' getters) may get in the way, but I think the SNI field has the same set-once property we rely on for the other getters.