ssl: parse SNI in received client hello records

The current implementation encodes SNI for client hellos sent, but does not decode those that are received. This patch decodes hostnames to the same format as the one used for encoding (the record named `sni`). I tested it by calling `tls_handshake:get_tls_handshake` on a fragment of a TLS 1.2 handshake, and it decoded the hostname.