share handshaking logic with QUIC

[marten-seemann]

Related issue: libp2p/go-libp2p-quic-transport#18.

The handshake logic is basically the same as in https://github.com/libp2p/go-libp2p-quic-transport/tree/identity.

[Stebalien]

Would it make sense to just make the quic transport depend on this package? It'll pull in some extra code but that would mean we wouldn't need an additional repo.

[marten-seemann]

Sounds reasonable. I just saw that I already exposed those functions, so we might even be able to do this without any further changes to this repo.

[marten-seemann]

Fixed by libp2p/go-libp2p-quic-transport#67.