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.
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
Since we already have `finished` channel to make the worker exit, we don't need to select
on `ctx.Done`, instead, we can only select `ctx.Done` in `Next`, and close the `finished` channel.
This avoid the cost to create context by `WithCancel` and select on extra channel.
