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/erasure-server-pool.go

@@ -1851,7 +1851,8 @@ func (z *erasureServerPools) ListBuckets(ctx context.Context, opts BucketOptions
 		listBucketsCache.Once.Do(func() {
 			listBucketsCache.TTL = time.Second
 
-			listBucketsCache.Relax = true
+			listBucketsCache.ReturnLastGood = true
+			listBucketsCache.NoWait = true
 			listBucketsCache.Update = func() ([]BucketInfo, error) {
 				ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 				buckets, err = z.s3Peer.ListBuckets(ctx, opts)