handle `RegionError.UndeterminedResult` in client-go

[lcwangchao]

UndeterminedResult is an new type error added to RegionError, see: pingcap/kvproto#1310

When client get this error, it should return ErrResultUndetermined to the caller without retry.

The outside code in TiDB should process the ErrResultUndetermined because the txn is in an undetermined state currently. For example, TiDB should disconnect the current connection at this time.