Use mysql2/promise to prevent problems where MySQL stracktraces are lost

As mentioned in https://github.com/sidorares/node-mysql2/issues/1067, this appears to be an issue with how Knex integreates with msyql2

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EDIT (by @mercmobily ): a possible path would be:

• Auto-Enable asyncStackTraces for MySQL2 in Development, write it in the DOCs
• Migrate to mysql2/promise under a flag OR with a different dialect (whichever is sanest)