Refactor: Move InstancePopulationAggregate calculations into the API

[Maelstromeous]

So currently the InstancePopulationAggregate forces the aggregator to be exclusive per census environment. This logic should be refactored out of the Aggregator and moved into the API.

The API can then scan the redis keys like it does in Aggregator and send the data to the database directly. Plus we have much better control over the timings of the data in the API project as well, it's kinda shoehorned into the Aggregator atm.

This also enables us to have multiple aggregators per environment, whereas right now if one crashes, no data can be processed until it's restarted.