`pdOracle` doesn't have retry mechanism like `KVStore`

[rleungx]

pdOracle:

client-go/oracle/oracles/pd.go

Lines 217 to 225 in 3150e38

	// GetTimestamp gets a new increasing time.
	func (o *pdOracle) GetTimestamp(ctx context.Context, opt *oracle.Option) (uint64, error) {
	ts, err := o.getTimestamp(ctx, opt.TxnScope)
	if err != nil {
	return 0, err
	}
	o.setLastTS(ts, opt.TxnScope)
	return ts, nil
	}

KVStore

client-go/tikv/kv.go

Lines 463 to 471 in 3150e38

	// CurrentTimestamp returns current timestamp with the given txnScope (local or global).
	func (s *KVStore) CurrentTimestamp(txnScope string) (uint64, error) {
	bo := retry.NewBackofferWithVars(context.Background(), transaction.TsoMaxBackoff, nil)
	startTS, err := s.getTimestampWithRetry(bo, txnScope)
	if err != nil {
	return 0, err
	}
	return startTS, nil
	}

If PD has a leader change, the first one will directly return the error instead of retrying.