Updated Object Store metrics in dashboards

[pracucci]

This PR updates objstore metrics reflecting changes done in cortexproject/cortex#2568. Some notes:

1. Created a new dashboard "Cortex / Object Store" to give an overview over all API calls to the store
2. Kept objstore panels in "Cortex / Reads" and "Cortex / Compactor", but added a per-operation QPS + per operation error rate (removed also "Op: Iter" because included in the per-operation QPS)
3. Removed the detailed objstore metrics from "Cortex / Writes" but added "Upload latency" to the shipper metrics row

Preview: Cortex / Reads

Preview: Cortex / Writes

Preview: Cortex / Object Store

[pstibrany]

LGTM!