lightning: let session vars in config also impact kv encoder

[glorv]

What problem does this PR solve?

Allow TiDB-Lightning explicitly set some session variables from config. In some situations, user may explicitly set time_zone in DM task config, so in the load stage, lightning should also use the same time_zone value to encode kv pairs or execute correspond SQL statements.

What is changed and how it works?

Proposal: xxx

What's Changed:

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:

• Little-Wallace
• kennytm

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[Little-Wallace]

LGTM

[glorv]

/merge

[ti-chi-bot]

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

Commit hash: 96c73e8

[ti-chi-bot]

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