raftstore: Avoid snapshot IO in raftstore thread (#16682)

[ti-chi-bot]

This is an automated cherry-pick of #16682

What is changed and how it works?

Issue Number: Close #16564

What's Changed:

Avoid snapshot IO in raftstore thread

This PR does:

• Only check snapshot meta file existence when receiving snapshot message so that don't need to read meta file content
• Don't calculate checksum when loading snapshot meta, because it would check in validate

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

Avoid snapshot IO in raftstore thread

[ti-chi-bot]

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