Proxy: prune Storage Gateway if other stores can provide the data quicker

[MichaHoffmann]

If we have a Querier that queries a Sidecar and a Storage Gateway that the Sidecar uploads to and if we query a range of data that could be completely answered by said Sidecar, we will still query the Storage Gateway regardless and throw away the data during deduplication. This is wasteful and also somewhat harmful since the Sidecar is usually orders of magnitude faster to query.
It would be great if Thanos could prefer serving data from the Sidecar in such a situation and prune the Storage Gateway.