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
}
```
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.
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.
