indexheader: call to Snapshotter.Stop must wait for its Start to finish

[narqo]

What this PR does

This one updates indexheader.Snapshotter in a way so the calls to Stop waited until the goroutine inside Start finished. This resolves occasional issues we see on the CI

--- FAIL: TestLabelValues_Cancelled (0.07s)
    bucket_test.go:2909: Creating 2 1-sample series with 1ms interval in /tmp/TestLabelValues_Cancelled3429144703/001/0
    bucket_test.go:2909: Creating 2 1-sample series with 1ms interval in /tmp/TestLabelValues_Cancelled3429144703/001/1
    testing.go:1231: TempDir RemoveAll cleanup: unlinkat /tmp/TestLabelValues_Cancelled3429144703/001: directory not empty

I think this is a simpler way to resolve the issue, and also I'm not sure #8434 actually solves it. I'm opening the PR just to remove extra burden from Marco.

Which issue(s) this PR fixes or relates to

Fixes #8428
Closes #8434

Checklist

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

[dimitarvdimitrov]

LGTM, I think I prefer this approach to #8434 because it also applies to regular shutdowns and solves the race condition

[pracucci]

I think it does the job. As a follow up experiment, I will try to convert Snapshotter into a services.Service to keep it more consistent with the rest of the code base.