`Locator.equals` so that locators can work properly in guava collections.
Preaggregated metric ingestion

* integration test (for later, when we can have those)
* `Conversions` for converting between stats, metrics and collections of each (collects functions that were spread around in different classes)
* Identify simple stat types in `StatType`.
* Link writer into ingestion pipeline.
* `TypeCacher` made synchronous