Boltdb shipper query performance improvements

[sandeepsukhani]

What this PR does / why we need it:
This PR includes a couple of improvements, all in a separate commit:

1. Uses new QueryWithCursor method which can reuse the same cursor to do all the index queries instead of having to start a new transaction for each query.
2. A new type called IndexDeduper which acts as a middleware for deduping the index entries which significantly improves the CPU usage and query performance.
3. Use Fifocache(an in-memory cache) in ingester which also significantly reduces the load on ingesters by caching the index in memory for serving subsequent queries. It also helps improve the query performance.
4. Snapshotting of dbs in ingesters for reads. This is again an optimization to improve query performance in ingesters. It would create a new snapshot of dbs in ingesters every minute for reads.
5. Ingesters now ship the files faster and retain fewer of them which reduces the amount of index ingesters have to deal with during queries. This also helps reduce the load on ingesters during queries.

All the changes complement each other to enhance query performance and reduce CPU usage. I have also fine-tuned some configs which have a supporting comment to explain why the change has been done.

Checklist

• Tests updated

[slim-bean]

LGTM!