syncer/dbconn: add more retry error (#1903)

[ti-chi-bot]

This is an automated cherry-pick of #1903

What problem does this PR solve?

Auto retry for mysql.ErrInvalidConn and tidb's kv.RetryableErr (by pingcap/tidb-tools#452).

close #1825

What is changed and how it works?

• When sql client interrupt (e.g. manually killed by kill tidb xxx), go-mysql will return mysql.ErrInvalidConn and this error is safe to auto-retry build the connection
• tidb's kv.RetryableErr (currently only caused by TxnLockNotFound) is retryable too.

Check List

Tests

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

Code changes

Side effects

Related changes

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

[GMHDBJD]

LGTM

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• GMHDBJD
• lance6716

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[lance6716]

/merge

[ti-chi-bot]

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

Commit hash: 0b7f218