Prevent further double vote incidents

[foriequal0]

Currently, there are no reliable double vote prevention mechanisms. Current VoteCollector constantly clears old votes, votes_received bitset is cleared every time we move_to_step. Even if we make a mistake (like we accidentally made a step regression, race condition, etc) the double vote shouldn't occur at any cost.

There should be a mechanism to assert an invariant (such as 1. no double vote on the same step, 2. vote step should be monotonically increased, etc.)

• Refactor to manage vote related states
• Add invariant
• Respect vote collector's double vote report