Improve the performance of `Sort` by parallel

Feature Request

Is your feature request related to a problem? Please describe:

Now the Sort executor is executed serially. We can run it parallelly to speed up.

Describe the feature you'd like:

Considering using a parallel framework, to run each sort partition in parallel. And then using a merge sort to generate the final result. We can refer to this proposal https://github.com/pingcap/tidb/pull/14238#issuecomment-569880893 and reuse it (after this pr merge).

Describe alternatives you've considered:

Task:

1. Implement parallel sort.

• Implement merge sort algorithm above Shuffle operator.
• Implement plan builder for parallel sort.

2. Cost Model, explain analyze information.

• Change the cost model of sort if it will run parallelly.
• Change the explain analyze result for parallel sort.

Teachability, Documentation, Adoption, Migration Strategy: