Support racing handshakes on undirected 5-tuples

[bemasc]

With DTLS and Connection IDs, it is possible in principle to establish connections with both polarities on a single 5-tuple simultaneously. With Datagram cTLS as currently specified, this does not work well, because a client who is expecting a ServerHello (CTLSServerPlaintext) might get another ClientHello (CTLSClientPlaintext) instead, and these structures rely on the directionality of the 5-tuple to determine the parsing.

We could add support for these kinds of racing handshakes by defining separate content types for CTLSServerPlaintext and CTLSClientPlaintext (at the cost of an additional registry entry), or by unifying these structures (at the cost of duplicating the profile_id).