local: retry remove on Windows sharing violation error #2202

Before this change asynchronous closes in cmount could cause sharing
violations under Windows on Remove which manifest themselves
frequently as test failures.

This change lets the Remove be retried on a sharing violation under
Windows.

backend/local/local.go

@@ -837,7 +837,7 @@ func (o *Object) lstat() error {
 
 // Remove an object
 func (o *Object) Remove() error {
-	return os.Remove(o.path)
+	return remove(o.path)
 }
 
 // Return the directory and file from an OS path. Assumes