feat(database): support multiple databases in migrations and query builders

[brendt]

No description provided.

[brendt]

@innocenzi I'd appreciate your input. I've left some clarification comments, and I'm aware there still are some todos

[innocenzi]

Pretty cool to see that we got to about the same conclusions, this looks a lot like my local changes 😼

I don't think having a UsesDatabase trait is that useful, its only purpose is to store a database variable with an awkward name that can conflict with internal properties.

I would go with a onDatabase method on the Query, and pass down Database instances from the query builders to the Query instance they create.

As for the BuildsQuery class, I think the name is fine as-is: it describes the intent of the interface very well, and the current QueryBuilder class is a builder class for building queries