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/data-usage.go

@@ -81,7 +81,8 @@ func loadPrefixUsageFromBackend(ctx context.Context, objAPI ObjectLayer, bucket
 		prefixUsageCache.TTL = 30 * time.Second
 
 		// No need to fail upon Update() error, fallback to old value.
-		prefixUsageCache.Relax = true
+		prefixUsageCache.ReturnLastGood = true
+		prefixUsageCache.NoWait = true
 		prefixUsageCache.Update = func() (map[string]uint64, error) {
 			m := make(map[string]uint64)
 			for _, pool := range z.serverPools {