[WIP]support multiple mysql hosts

[shonge]

What problem does this PR solve?

resolved #1013

What is changed and how it works?

support mysql downstream
[syncer.to]
host = "192.168.0.1,192.168.0.2"

Randomly choose one of the hosts as downstream mysql host

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Code changes

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

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

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

[ti-chi-bot]

@shonge: PR needs rebase.

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