fs: Enable returning ETag along with ListObjects() (#4042)

This is to comply with S3 behavior, we previously removed
reading `fs.json` for optimization reasons but we have a
reason to believe that providing ETag and using gjson
provides needed benefit of not having to deal with
unmarshalling overhead of golang stdlib.

Fixes #4028

cmd/test-utils_test.go

@@ -67,6 +67,9 @@ func init() {
 
 	// Set system resources to maximum.
 	setMaxResources()
+
+	// Quiet logging.
+	log.logger.Hooks = nil
 }
 
 func prepareFS() (ObjectLayer, string, error) {