Optimize adding index by remove unnecessary locking

[wjhuang2016]

Enhancement

tidb/ddl/index.go

Lines 1332 to 1341 in 9c683d3

	// Lock the row key to notify us that someone delete or update the row,
	// then we should not backfill the index of it, otherwise the adding index is redundant.
	err := txn.LockKeys(context.Background(), new(kv.LockCtx), idxRecord.key)
	if err != nil {
	return errors.Trace(err)
	}
	// Create the index.
	handle, err := w.index.Create(w.sessCtx, txn, idxRecord.vals, idxRecord.handle, idxRecord.rsData)
	if err != nil {

We don't need to lock the key when adding an index. And it can cause hot-point.