branch prediction optimization in MySQLCommandExecutorFactory and MySQLCommandPacketFactory

[lujx98]

Changes proposed in this pull request:

• In my opinion, considering in our actual env,most of our commands are "COM_QUERY".I think it may improve perfomance if we extract the COM_QUERY branch from switch statement

detail: https://dubbo.apache.org/zh-cn/blog/2019/02/03/%E6%8F%90%E5%89%8Dif%E5%88%A4%E6%96%AD%E5%B8%AE%E5%8A%A9cpu%E5%88%86%E6%94%AF%E9%A2%84%E6%B5%8B/#

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[linghengqian]

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