Validators are wrongly sorted for v.0.34

[andrey-kuprianov]

There is an issue with the current sorting of validators which I've discovered because my MBT tests were failing:

In v.0.34 the validators should be first sorted by voting power (descending), then by address (ascending), but here they are sorted only by voting power (see #506). As a result, header hash validation fails.

The issue is actually also that it's not properly documented in the tendermint/spec repo and in the tendermint-rpc, the requirement is only implemented in tendermint-go.

I've already stumbled upon this before, but forgot to open the issues against the documentation repos (my bad!); will do that now. Will also fix the issue in this repo.