refers to ticket #457

- does aggmetric call the cache push callback every time a metric gets
  evicted from the ring buffer?
- does aggmetric add the chunk into the store if the node is primary?
- makes the devnullStore count how many times it's Add() got called
- does aggmetric not add the chunk into the store if the node is not primary?

this lets us simplify the unit tests and focus them on their business
logic