Copy metadata before spawning goroutine + prealloc maps (#10458)

In `(*cacheObjects).GetObjectNInfo` copy the metadata before spawning a goroutine.

Clean up a few map[string]string copies as well, reducing allocs and simplifying the code.

Fixes #10426

cmd/object-api-options.go

@@ -168,7 +168,7 @@ func putOpts(ctx context.Context, r *http.Request, bucket, object string, metada
 	etag := strings.TrimSpace(r.Header.Get(xhttp.MinIOSourceETag))
 	if etag != "" {
 		if metadata == nil {
-			metadata = make(map[string]string)
+			metadata = make(map[string]string, 1)
 		}
 		metadata["etag"] = etag
 	}