Instead of having one struct with uninitialised fields
which are mutated, have explicit states and ensure that the types
show that there is no ambiguity which data is available or not.
Consequently, this removes quite a few `unwrap`/`expect` calls.
* Introduce NULL cipher and allow more configuration.
* Back to using the hash-code for handshake.
Using `Endpoint` would be incompatible with the existing protocol.
* Add comments.
