[cryptography] explore making DKG less error-prone to use

[cronokirby]

There are a good amount of moving parts in the DKG outside of the code itself, which doesn't provide strong guard-rails in terms of usage.

In particular, the role of the arbiter, and the need to sign certain messages, etc. is unclear.

It might be useful to try and create an interface which is much less prone to error, and handles more of the cryptographically-sensitive logic internally.

c.f. #1530 for an example (imo, successful) of lifting up this kind of logic.