Introduce a base() method to get the baseExecutor, so we don't need to add
method from baseExecutor to the Executor interface any more
Before:
```
type Executor interface {
Open(context.Context) error
Next(ctx context.Context, req *chunk.RecordBatch) error
Close() error
Schema() *expression.Schema
retTypes() []*types.FieldType
newFirstChunk() *chunk.Chunk
}
```
After:
```
type Executor interface {
base() *baseExecutor
Open(context.Context) error
Next(ctx context.Context, req *chunk.RecordBatch) error
Close() error
Schema() *expression.Schema
}
```
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
