Quick support to server level WORM (#5602)

This is a trival fix to support server level WORM.  The feature comes
with an environment variable `MINIO_WORM`.

Usage:
```
$ export MINIO_WORM=on
$ minio server endpoint
```

cmd/bucket-handlers.go

@@ -300,6 +300,14 @@ func (api objectAPIHandlers) DeleteMultipleObjectsHandler(w http.ResponseWriter,
 		return
 	}
 
+	// Deny if WORM is enabled
+	if globalWORMEnabled {
+		// Not required to check whether given objects exist or not, because
+		// DeleteMultipleObject is always successful irrespective of object existence.
+		writeErrorResponse(w, ErrMethodNotAllowed, r.URL)
+		return
+	}
+
 	var wg = &sync.WaitGroup{} // Allocate a new wait group.
 	var dErrs = make([]error, len(deleteObjects.Objects))