Dedup ExpandPostings call

[yeya24]

Is your proposal related to a problem?

Similar to #7169, duplicate requests of expanding postings for the same matchers, can consume a lot of bandwidth and CPU.

Describe the solution you'd like

Introduce https://pkg.go.dev/golang.org/x/sync/singleflight to https://github.com/thanos-io/thanos/blob/main/pkg/store/bucket.go#L1099 to dedup ExpandPostings requests.

It is not enough to just do #7169 because #7169 only dedups requests to cache. However, postings insection/merge operations are still duplicated.