validators api: use validator history to find inactive validators

[pro-wh]

Currently the /validators endpoint will treat every entity that does not have an active node as an inactive validator. We want to filter this to only return entities that have been a validator at some point in the past, but we do not store this historical data yet.

See this comment thread for more info.

703 should add this historical validator data, which we can then use to filter entities appropriately. Once it is merged, update the /validators api endpoint. 703 may store a set of all past/current validators in the chain.epochs table, which would be a good starting point for the ValidatorsData join query.