* 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
We import "github.com/pkg/errors" in the code, and actually put
the "github.com/pingcap/errors" there in the vendor path.
That's a dirty hack, and prevent the introduce of Go module.
Do not modify Plan of dataReaderBuilder directly, because it would
impact next batch of outer rows, as well as other concurrent inner
workers. Instead, build a local child builder to store the child plan.
Thank you for working on TiDB! Please read TiDB's [CONTRIBUTING](https://github.com/pingcap/tidb/blob/master/CONTRIBUTING.md) document **BEFORE** filing this PR.
## What have you changed? (mandatory)
During coprocessor dag task, it first uses timezone `name`, if non-empty, to get legitimate timezone variable. To achieve this, we need to push down such data into tikv which leads to change the logic of building pushdown request. The logic I mentioned mainly resides in `executor` package.
I change `timeZoneOffset` to `zone` and add second return parameter `name string`. The intentioned of doing this to adopt the convention of `time` package.
For the same purpose, I change `GetTimeZone` to `Location`. As you can see, in `time` package, timezone was bind to `Location`.
## What are the type of the changes (mandatory)?
- Bug fix (non-breaking change which fixes an issue)
## How has this PR been tested (mandatory)?
unit-test
integration-test will be added shortly
