merge MySQL to upstream preview

[alswl]

终于搞定了冲突，@agapple 看看。

这个版本不能直接合并到上游，由于单测覆盖率低，我害怕合并过程中产生额外问题。
麻烦先看看特性和功能，再酌情取特性分支合并。

[CLAassistant]

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[CLAassistant]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ alswl
❌ shilin

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[alswl]

结构调整介绍的文章：https://blog.alswl.com/2018/06/sql-server-migration-3/