Use libsignal protocol

[okdistribute]

@ept did a demo of spake2 with libsignal protocol, which is more secure by adding forward secrecy. It would introduce a key per device which has some usability & security implications for multidevice that I could go into more detail on, still researching what those are exactly. https://github.com/ept/spake2-signal/blob/main/index.html