fix(consensus): support validators size change

[oXtxNt9U]

Summary

Support changing the validator count via milestones.

The main issue is that the consensus uses inconsistent milestones while it needs to use the milestones for the proposed height.

Milestone changes are now logged differently to make debugging easier:

Milestone change: {"height":"0 => 4","activeValidators":"3 => 7","reward":"0 => 200000000"}
Milestone change: {"height":"4 => 11","activeValidators":"7 => 4"}

Also adds TODOs in places that are potentially using the wrong milestones too, but are out of scope for this PR.

Checklist

• Documentation (if necessary)
• Tests (if necessary)
• Ready to be merged