[noise] half-duplex protocols

[David Wong]

In the context of half-duplex protocols were the initiator and the
responder are taking turns to send messages on the channel, it would
make sense not to use a Split function but continue to use the
CipherState as is.

The Split function is called in WriteMessage and ReadMessage at the very end:

> If there are no more message patterns returns two new CipherState objects by calling Split().

I'm not sure how to modify this, maybe just as an advanced feature?

(I'm asking in the context of the Disco extension where Strobe works
really well for half-duplex protocols)

David