Don't copy interesting ids, clean up logging (#11102)

When searching the caches don't copy the ids, instead inline the loop.

```
Benchmark_bucketMetacache_findCache-32    	   19200	     63490 ns/op	    8303 B/op	       5 allocs/op
Benchmark_bucketMetacache_findCache-32    	   20338	     58609 ns/op	     111 B/op	       4 allocs/op
```

Add a reasonable, but still the simplistic benchmark.

Bonus - make nicer zero alloc logging

cmd/metacache-bucket_test.go

@@ -0,0 +1,55 @@
+package cmd
+
+import (
+	"fmt"
+	"testing"
+)
+
+func Benchmark_bucketMetacache_findCache(b *testing.B) {
+	bm := newBucketMetacache("", false)
+	const elements = 50000
+	const paths = 100
+	if elements%paths != 0 {
+		b.Fatal("elements must be divisible by the number of paths")
+	}
+	var pathNames [paths]string
+	for i := range pathNames[:] {
+		pathNames[i] = fmt.Sprintf("prefix/%d", i)
+	}
+	for i := 0; i < elements; i++ {
+		bm.findCache(listPathOptions{
+			ID:           mustGetUUID(),
+			Bucket:       "",
+			BaseDir:      pathNames[i%paths],
+			Prefix:       "",
+			FilterPrefix: "",
+			Marker:       "",
+			Limit:        0,
+			AskDisks:     0,
+			Recursive:    false,
+			Separator:    slashSeparator,
+			Create:       true,
+			CurrentCycle: uint64(i),
+			OldestCycle:  uint64(i - 1),
+		})
+	}
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		bm.findCache(listPathOptions{
+			ID:           mustGetUUID(),
+			Bucket:       "",
+			BaseDir:      pathNames[i%paths],
+			Prefix:       "",
+			FilterPrefix: "",
+			Marker:       "",
+			Limit:        0,
+			AskDisks:     0,
+			Recursive:    false,
+			Separator:    slashSeparator,
+			Create:       true,
+			CurrentCycle: uint64(i % elements),
+			OldestCycle:  uint64(0),
+		})
+	}
+}