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/metrics-v2.go

@@ -357,7 +357,7 @@ type MetricsGroupOpts struct {
 func (g *MetricsGroup) RegisterRead(read func(ctx context.Context) []Metric) {
 	g.metricsCache = cachevalue.New[[]Metric]()
 	g.metricsCache.Once.Do(func() {
-		g.metricsCache.Relax = true
+		g.metricsCache.ReturnLastGood = true
 		g.metricsCache.TTL = g.cacheInterval
 		g.metricsCache.Update = func() ([]Metric, error) {
 			if g.metricsGroupOpts.dependGlobalObjectAPI {