migrate web::releases to sqlx, set up sqlx db pool for production

[syphar]

This migrates the first batch of handlers to sqlx / async database.
I introduced a second setting for the async pool size so be able to update the sizes depending on the errors or usage.

Generally I don't like having block_on in the test-case, I would prefer using tokio::test (or sqlx::test) for that, but that needs more refactoring.