Add IndexLookUp executor and extra output channels

[lcwangchao]

Signed-off-by: Chao Wang cclcwangchao@hotmail.com

What problem does this PR solve?

Issue Number: close #375

Problem Summary:

To support IndexLookUp push-down, we should introduce some new fields in tipb

What is changed and how it works?

1. Add a new executor IndexLookUp with type ExecType_TypeIndexLookUp
2. After IndexLookUp is introduced the DAGRequest.executors should be able to represent a tree instead of a simple array. We explained how executors orders in this PR (compatible with the previous case when each parent has at most one child):

// left child first, then right child, then parent.
// For example, a DAG:
//                A
//               /
//              B
//            /   \
//           C     D
//          /     / \
//         E     F   G
//        /
//       H
// Its order should be: [H, E, C, F, G, D, B, A]

3. Add a new field parent_idx to Executor to help to parse the DAG.
4. Add IntermediateOutputChannel and IntermediateOutput for intermediate results.

Related changes

The main tracking issue is: pingcap/tidb#62575

[pingcap-cla-assistant]

All committers have signed the CLA.