[ES-1314123] race condition for memorize tsdb client

[jnyi]

Revert thanos-io#7782

Also submit one to OSS: thanos-io#7941

Add this to test TestMultiTSDBAddNewTenant would fail in current OSS code:

	concurrency := 100
	var wg sync.WaitGroup
	for i := 0; i < concurrency; i++ {
		wg.Add(1)
		go func(i int) {
			defer wg.Done()
			testutil.Ok(t, appendSample(m, fmt.Sprintf("tenant-%d", i), time.UnixMilli(int64(10))))
		}(i)
                 go func() {
			m.TSDBLocalClients()
		}()
	}
	wg.Wait()
	testutil.Equals(t, 1+concurrency, len(m.TSDBLocalClients()))

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

Verification

[yuchen-db]

great work! thanks