Replace mysql with mysql2

[goloroden]

What is this task about?

The mysql npm package does not seem to be alive any more. Hence we should migrate to mysql2, especially given that it supports a promise-based API out of the box. However, it is missing official @types support. So we may need to work around this.

What needs to be done to complete this task?

• Replace the npm dependency
• Switch to the promise-based API
• Add missing types (and maybe contribute back)

What else should we know?

This is needed to prepare support for MySQL 8.

[goloroden]

Good news on the mysql package, apparently something is going on there, and there should be a new version with support for MySQL 8 in early 2020 (see mysqljs/mysql#2233 (comment) for details).

Hence, let's hope the best, and let's close this issue for now.