Originally, `fail` is added to simplify error handling, avoiding large changes on
the Txn() interface API. But it turns out that `fail` becomes the root of many bugs.
So this commit remove the `fail` and make an API refactory.
* change Txn() function signature to Txn(active bool)
* ActivePendingTxn() is not used any more because Txn() does the work
* change executor builder getStartTS() uint64 to getStartTS() (uint64, error)
`set @@autocommit = 0`
`select 1` // This statement should not make the session enter a transaction,
because it never need Txn().
Make txn lazy initialize
