Measure and visualise requests per storage provider

[bajtos]

See the discussion in CheckerNetwork/spark-checker#50

To help us understand how many request we send to each storage provider, we should add a new retrieval stat and visualise it in Grafana.

We cannot associate retrievals with a storage deal (and a miner id) now. Until that happens (see #65), let's use the provider's address as a proxy representing a unique SP.

[bajtos]

Closing as done. We have per-miner RSR stats, they include total number of retrieval checks performed by the network.