Make transaction life cycle under control, prepare for correctly handle schema out of date error. The old `GetTxn(forceNew boll)` is replaced by `Txn` and `NewTxn`, `CommitTxn` and `RollbackTxn` is removed.
Make transaction life cycle under control, prepare for correctly handle schema out of date error. The old `GetTxn(forceNew boll)` is replaced by `Txn` and `NewTxn`, `CommitTxn` and `RollbackTxn` is removed.