Limit proxy's memory from TiFlash's side (#408)

[CalvinNeo]

This is an manual cherry-pick of #408

What is changed and how it works?

Issue Number: Close tiflash#9745

What's Changed:

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

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

Release note

Fix an issue that TiFlash may unexpectedly reject raft message when memory usage is not high

[CalvinNeo]

/retest

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This pull-request has been approved by: JaySon-Huang

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