proxy: Decouple prehandle snapshot from RegionWorker

[CalvinNeo]

Signed-off-by: CalvinNeo [email protected]

What problem does this PR solve?

Issue Number: ref #5170

Problem Summary:

TiFlash Proxy used to reuse RegionWorker to prehandle snapshot. In this PR, we move the prehandle and apply snapshot procedure out of raftstore to our observer. The whole process remain the same, excluding:

1. The prehandle pool is owned by observer
2. We introduce two raftstore configs
3. Proxy will do what a TiKV will do now, except for real ingesting.

A simple "flow control" mechanism which aims to make full utilization of pre handle thread pool is still in raftstore and will be removed in the future.

We also use a independent branch to test this.

What is changed and how it works?

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]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• breezewish
• lidezhu

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[CalvinNeo]

/merge

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: d433530

[sre-bot]

Coverage for changed files

no c/c++ source change detected

Coverage summary

Functions  MissedFunctions  Executed  Lines   MissedLines  Cover
18422      8328             54.79%    213050  85975        59.65%

full coverage report (for internal network access only)