executor: change sort to parallel

[yu34po]

What problem does this PR solve?

What is changed and how it works?

make sort executor to parallel process
how it works:
1.get all rows from children executor
2.start several sort worker goroutine, average split rows to every sort worker, for every sort worker sort rows of itself
3.merge the sorted rows from every sort worker

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Code changes

• Has exported function/method change
• Has exported variable/fields change

Side effects

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to update the tidb-ansible repository
• Need to be included in the release note

[codecov-io]

Codecov Report

Merging #9418 into master will increase coverage by 0.03%.
The diff coverage is 88.66%.

@@            Coverage Diff             @@
##           master    #9418      +/-   ##
==========================================
+ Coverage   67.42%   67.45%   +0.03%     
==========================================
  Files         373      374       +1     
  Lines       78564    78719     +155     
==========================================
+ Hits        52968    53097     +129     
- Misses      20839    20858      +19     
- Partials     4757     4764       +7

Impacted Files	Coverage Δ
executor/executor.go	68.16% <ø> (ø)	⬆️
sessionctx/variable/session.go	30.47% <100%> (+0.22%)	⬆️
executor/builder.go	83.79% <100%> (ø)	⬆️
executor/merge_sort.go	88.51% <88.51%> (ø)
ddl/delete_range.go	75.13% <0%> (-4.24%)	⬇️
expression/schema.go	93.75% <0%> (-0.79%)	⬇️
ddl/ddl_api.go	77.21% <0%> (+0.05%)	⬆️
executor/show_stats.go	89.92% <0%> (+0.14%)	⬆️

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[XuHuaiyu]

Should we add a definition for this in sessionctx/variable/sysvar.go

[zz-jason]

@yu34po Sorry for the late review, please update to the master branch and resolve conflicts.

[sre-bot]

Hi contributor, thanks for your PR.

This patch needs to be approved by someone of admins. They should reply with "/ok-to-test" to accept this PR for running test automatically.

[zz-jason]

closed due to outdated, fell free to reopen it if it's needed.