chore: using mysql2 as library for serverless-mysql

[andreabadesso]

Motivation

The last update of the mysql library we were using was in Jan 23, 2020

Acceptance Criteria

• We should use mysql2 as the library for serverless-mysql (mysql is the default)
• We should remove the mysql dependency

Security Checklist

• Make sure you do not include new dependencies in the project unless strictly necessary and do not include dev-dependencies as production ones. More dependencies increase the possibility of one of them being hijacked and affecting us.

[andreabadesso]

mysql2 is returning SUM, MIN and MAX as strings, so we can either cast it on mysql itself or on javascript, with parseInt, I prefer doing it directly in mysql

[r4mmer]

Casting to integer is fine but the SUM returns a DECIMAL (which the client converts to a string) because of a posible overflow and precision issues. Maybe we should check no cast has this problem, like a sum of output values that can go over the mysql native integer highest number.

[andreabadesso]

The last bump didn't update this, totally unrelated to the PR

[r4mmer]

There are some warnings like "Argument 'mysql' should be typed" which can be fixed if we use the mysql2 types

[r4mmer]

Casting to integer is fine but the SUM returns a DECIMAL (which the client converts to a string) because of a posible overflow and precision issues. Maybe we should check no cast has this problem, like a sum of output values that can go over the mysql native integer highest number.