Refactor Validator Sets

[brentstone]

As specified in #283, the validator sets should be refactored from active and inactive to consenses, below_capacity, and below_threshold. The active and consensus validator sets are the same (just renaming).

The inactive validator set is further refactored into below_capacity and below_threshold. The "threshold" refers to a level of bonded stake below which we do not iterate over the validators in various PoS actions. The goal is to prevent unbounded iteration over validator sets.

Currently based on #714