Store-gateway memory usage

In a large scale deployment, store-gateway service is prone to OOM issues due to the way we handle TSDB indices: once block is opened, its TSDB index is loaded in memory and never released until shutdown.

The issue is aggravated by the fact that the index:
- is not sharded: all series of a tenant block are stored in a single blob
- is not optimized for reading from the object storage: the whole index is to be loaded into memory before access

This makes it hard to cache the index efficiently: even if we pulled it from some very fast cache, we'd still need to have enough memory to keep indexes affected by the query in memory