Remove unnecessary wait for MPP tunnel destruction before shutdown (#10313)

[ti-chi-bot]

This is an automated cherry-pick of #10313

What problem does this PR solve?

Issue Number: ref #10266

Problem Summary:

What is changed and how it works?

After #10299, TiFlash will wait for all MPPTasks to finish before shutdown.

Whether all MPP tasks have finished is determined by checking if monitored_tasks is empty. In MPPTask, MPPTaskMonitorHelper is the last member to be destructed, and only when it is destructed will removeMonitoredTask be called to remove the task id from monitored_tasks. MPPTunnels are stored in MPPTunnelSet, and although MPPTunnelSet is held by a shared_ptr, there are actually only two references: one directly in MPPTask, and the other inside the std::unique_ptr<DAGContext> also in MPPTask. Therefore, destroying the MPPTask will also destroy the MPPTunnel.

Remove unnecessary wait for MPP tunnel destruction during TiFlash shutdown

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

None

[ti-chi-bot]

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[ti-chi-bot]

@gengliqi This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

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