Add "handshake seal"

[yusefnapora]

libp2p/specs#234 adds a new requirement to the spec, which we should implement here.

To prevent replay attacks & tampering of handshake messages, we sign the Noise HandshakeHash with the libp2p key and send the signature before any ciphertexts are exchanged over the secure channel. Each party will verify the signature against their own HandshakeHash, which proves that both parties have the same view of the handshake exchange and that nothing has been altered.

[yusefnapora]

Oh, also, we should update the protobuf field names to match the changes in libp2p/specs#234 while we're at it.