Decoupled BucketStores from UserStore

[pracucci]

What this PR does:
The upcoming store-gateway (proposal) will run the Thanos BucketStore instead of the querier.

Currently, we do have a multi-tenant BucketStore wrapper which is UserStore. The UserStore is both a wrapper over BucketStore and a service which does an initial blocks synching + a periodic sync.

The store-gateway will need to trigger the sync differently due to the sharding. In particular, the store-gateway needs to coordinate the initial sync while the instance is in the JOINING state within the ring (because it needs to have the tokens first) and to trigger a sync both periodically and when the token ranges assigned to instance itself change (ring topology changed).

Instead of duplicating code between the querier and store-gateway for the co-existence period, in this PR I'm decoupling the multi-tenant wrapper of BucketStore from UserStore:

• BucketStores: multi-tenant wrapper over BucketStore (will be directly used by the store-gateway). The Series() API here matches the exact gRPC service signature of the upcoming store-gateway so that we can do a pass-through to the Thanos BucketStore
• UserStore: BucketStores + service. I haven't renamed it in this PR to keep the diff easier. Also UserStore will disappear once the store-gateway will be complete.

Which issue(s) this PR fixes:
N/A

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[pstibrany]

LGTM.

UserStore: BucketStores + service. I haven't renamed it in this PR to keep the diff easier. Also UserStore will disappear once the store-gateway will be complete.

Feel free to rename, it's in its own file now anyway, diff will not be much bigger I think.

Unrelated to this PR, but perhaps worth fixing... should function used in Iter call react on context being done?

		select {
		case jobs <- job{userID: user, store: bs}:
			return nil
		case <-ctx.Done():
			return ctx.Err()
		}

Otherwise, it can be blocked here for some time, until some worker is finished with sync.

[pstibrany]

I don't think it will. I think deduplicate filter removes old blocks if new blocks cover it completely, but we better verify it.

[pracucci]

I think we may have troubles with the consistency check, at least as I did design it. I will write down different scenarios in tests and let's see (once we'll reach the point to build the consistency check).

[pracucci]

Unrelated to this PR, but perhaps worth fixing... should function used in Iter call react on context being done?

Definitely.

[pracucci]

Feel free to rename, it's in its own file now anyway, diff will not be much bigger I think.

As agreed offline, renamed UserStore to BucketStoresService.