tracker: get some session variables from downtream #1032

[lance6716]

manually cherry-pick #1032

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What problem does this PR solve?

enable some test about altering PK, and reduce user config. close #1003

What is changed and how it works?

• session variable:
  • if user specified, use it
  • else get sql_mode, tidb_skip_utf8_check from downstream
• tracker (tidb) config:
  • set alter-primary-key true
• max-allow-packet
  • always get from downstream (by feature of driver)

Check List

Tests

• Unit test
• Integration test

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has persistent data change

Side effects

• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to be included in the release note

[CLAassistant]

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2 out of 3 committers have signed the CLA.

✅ csuzhangxc
✅ lance6716
❌ ti-srebot
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