Integration test: return a 422 status code when store-gateway and quierier hit the max-fetched-series-per-query limit

[duricanikolic]

What this PR does

This PR introduces the following changes:

• store_gateway_client.go and store_gateway_test.go have been moved from the querier to the storegateway package
• Struct StoreGatewayClient from store_gateway_client.go has been renamed to StoreGatewayClientImpl, because it actually implements the homonymous interface belonging to the same package
• New integration tests have been added in store_gateway_limits_hit_test.go: they ensure that when the max-fetched-ser-es-limit is hit, both store-gateway and querier will return a 422 status code

Which issue(s) this PR fixes or relates to

Extension of the fix for issue #3382

Checklist

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

[pracucci]

store_gateway_client.go and store_gateway_test.go have been moved from the querier to the storegatewaypb package

The *pb packages should contain only protobufs.

That being said I don't think an integration should use a gRPC client. The idea of integration tests is that they should emulate how users use the system. Let's talk on Slack about it, so we can see if we can find a better approach for the testing here.

[duricanikolic]

The *pb packages should contain only protobufs. @pracucci I have fixed that and moved the 2 sources to the storegateway package instead.

For the rest, let's discuss it next week after you have reviewed the code.