Replace r2d2 (non-async) with sqlx (async)

[mickvandijke]

[josecelano]

Hi @WarmBeer @da2ce7, we should consider more alternatives like:

• https://diesel.rs/ (mysql, postgresql, sqlite)
• https://github.com/prisma/quaint (mysql, postgresql, sqlite, mssql)

I think we should decide first which features we want to have:

• ORM
• DB migrations
• Support for other databases, not only mysql and sqlite
• Performance
• Async
• ...

I do not think having support for multiple DBs or an ORM is necessary for this project.

I'm assuming:

• The DB schema is not going to change often, but DB migrations are still convenient.
• The persistence is minimal.

From my point of view, I would:

• Switch to sqlx if it's faster.
• Include DB migrations.
• Split the Database trait into more independent repositories, like the contexts we have for the API: stats, torrents, whitelist, auth_keys.