workerpool: remove mergeContext (#2201)

[amyangfei]

This is an automated cherry-pick of #2201

What problem does this PR solve?

• mergeContext spawns a goroutine when Done() is called for the first time. Since we generate a mergeContext for each event before calling its handler, when concurrency is high, we will be running a large number of goroutines concurrently, and these goroutines do little that's useful.

Close #2211

What is changed and how it works?

• Remove mergeContext completely. Event handler calls can only be cancelled with the ctx used to add that event. They will no longer be cancelled when the workerpool is cancelled.

Check List

Tests

• Unit test
• Integration test

Related changes

• Need to cherry-pick to the release branch

Release note

• Optimize workerpool for fewer goroutines when concurrency is high.

[ti-chi-bot]

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[ti-chi-bot]

@amyangfei: Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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[purelind]

/run-all-tests

[amyangfei]

/run-all-tests /tikv=release-5.2

[amyangfei]

Close this PR since it will be picked in https://github.com/pingcap/ticdc/pull/2490