Remove sqlx dependency?

[cdevienne]

The only reason sqlx is still a dependency is to be able to use sqlx features on the underlying connection.
I wonder if we could achieve that by optionally injecting the db connection in qb instead of having qb create it, and provide an interface for

For someone wanting to use sqlx features, the engine initialization would look like:

    db, err := sqlx.Connect("postgres", "user=foo dbname=bar sslmode=disable")
    engine, err := qb.NewEngine("postgres", db)

Since we never use Queryx in qb, the only remaining problem would be transaction handling.
We currently have to use db.Beginx() to keep sqlx compat.

I don't see a clean way to handle this for now, but I would prefer such dependencies to be optional.