lightning: generate and send region job more smoothly (#42780)

[ti-chi-bot]

This is an automated cherry-pick of #42780

What problem does this PR solve?

Issue Number: close #42456

Problem Summary:

What is changed and how it works?

1. in the old PR we try to generate all region jobs at once, then process jobs, and generate jobs for unfinished range again, then process again... In order to reduce the lag caused by generating, we now start processing jobs once a key range has generated job, and start generating job once the job need to be generated again, like the region is changed. Also, these work are done concurrently.
2. unfinishedRange is removed because we don't have a barrier to stop all processing before use unfinishedRange to generate job. Now the after the job is initially generated, the key range of job will be totally finished by worker, or shrank by partitially ingestion. The job may also be splitted into multiple job whose total key range equals to the original job.
3. In order to let the workers schedule the backoffing job for later processing, a regionJobRetryer is introduced. Now the workflow becomes

	/*
	   [prepareAndSendJob]-----jobToWorkerCh--->[workers]
	                        ^                       |
	                        |                jobFromWorkerCh
	                        |                       |
	                        |                       v
	               [regionJobRetryer]<--[dispatchJobGoroutine]-->done
	*/

Reviews please help check if there's deadlock and if there's a better way to implement it.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

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[ti-chi-bot]

@ti-chi-bot: This cherry pick PR is for a release branch and has not yet been approved by release team.
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[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/check_dev	16781a8	link	true	/test check-dev
idc-jenkins-ci-tidb/mysql-test	16781a8	link	true	/test mysql-test
idc-jenkins-ci-tidb/check_dev_2	16781a8	link	true	/test check-dev2
idc-jenkins-ci-tidb/build	16781a8	link	true	/test build
idc-jenkins-ci-tidb/unit-test	16781a8	link	true	/test unit-test

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