Improve caching (#19130)

* Remove lock for cached operations.
* Rename "Relax" to `ReturnLastGood`.
* Add `CacheError` to allow caching values even on errors.
* Add NoWait that will return current value with async fetching if within 2xTTL.
* Make benchmark somewhat representative.

```
Before: BenchmarkCache-12       16408370                63.12 ns/op            0 B/op
After:  BenchmarkCache-12       428282187                2.789 ns/op           0 B/op
```

* Remove `storageRESTClient.scanning`. Nonsensical - RPC clients will not have any idea about scanning.
* Always fetch remote diskinfo metrics and cache them. Seems most calls are requesting metrics.
* Do async fetching of usage caches.

cmd/admin-handlers-users.go

@@ -1203,7 +1203,8 @@ func (a adminAPIHandlers) AccountInfoHandler(w http.ResponseWriter, r *http.Requ
 		bucketStorageCache.TTL = 10 * time.Second
 
 		// Rely on older value if usage loading fails from disk.
-		bucketStorageCache.Relax = true
+		bucketStorageCache.ReturnLastGood = true
+		bucketStorageCache.NoWait = true
 		bucketStorageCache.Update = func() (DataUsageInfo, error) {
 			ctx, done := context.WithTimeout(context.Background(), 2*time.Second)
 			defer done()