improve quality of vote_collector module

[debris]

I was just reading the code to understand what's going on over there and added several insubstantial improvements

[rphmeier]

I think eventually we can replace this module with the implementation here for vote accumulation. https://github.com/paritytech/polkadot/blob/master/substrate/bft/src/generic/accumulator.rs

There are a few small improvements like a generalization of justifications (to support stuff like BLS-threshold signatures) and misbehavior generation, but otherwise it serves the same purpose in a (IMO) clearer way.

FWIW that entire module contains a futures-based implementation of something pretty close to tendermint, so if we had generate_seal return a future (desired for AuraV2 also) we could probably just plug the whole thing in.

[debris]

@rphmeier I created a new issue for that ;)

[andresilva]

LGTM!