Store: make index cache shared cache friendly

[pracucci]

The IndexCache is an in-memory cache whose exposed interface allow to cache single Postings and Series. In Cortex we've an experimental TSDB blocks storage integration based on Thanos and we would like to introduce a shared index cache (based on Memcached).

The current IndexCache interface is not shared cache friendly, due to the fact that the exposed functions allow to fetch/store single items. In this PR I'm proposing to switch IndexCache to an interface which allows to fetch multiple postings and series at once, in order to make it shared cache friendly.

Fixed #1815

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

• Renamed store cache.go to inmemory.go
• Refactored IndexCache to multiple fetch postings and series within a single call
• Prefixed fetch/store functions with Fetch and Store for clarity

Due to the file renaming, the diff is quite difficult to follow, reason why I've splitted into two separate commits (it should help).

Verification

Unit and manual tests.

[bwplotka]

LGTM!

Super elegant code, just one minor nit for comment on exported method useful for goDoc (: Otherwise happy to merge.

[pracucci]

Thanks @bwplotka for your review! What's about now?

[bwplotka]

Thanks!