Fix: PostingCache promise should fetch data only once

[alanprot]

What this PR does:
Follow up of #6296

Under concurrency, we are fetching the postings multiples times for the same key.

The reason for that is we load the previous promise from the cache but the promise does not have the ts set yet and so, we assume is expired:

See:

cortex/pkg/storage/tsdb/expanded_postings_cache.go

Lines 320 to 332 in 2e5488a

	loaded, ok := c.cachedValues.LoadOrStore(k, r)
	if !ok {
	r.v, r.sizeBytes, r.err = fetch()
	r.sizeBytes += int64(len(k))
	r.ts = c.timeNow()
	c.created(k, r.sizeBytes)
	c.expire()
	}
	// If is cached but is expired, lets try to replace the cache value
	if ok && loaded.(*cacheEntryPromise[V]).isExpired(c.cfg.Ttl, c.timeNow()) {
	if c.cachedValues.CompareAndSwap(k, loaded, r) {

This PR is simplifying the implementation and waiting the loaded promise to finish before checking if is expired.

First commit of the PR has only the test showing the problem.

Which issue(s) this PR fixes:
Fixes #

Checklist

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

[yeya24]

Thanks. Make sense to me