GRDB 7: MainActor ValueObservation scheduling

[groue]

The default ValueObservation scheduling is now .mainActor, which has database change callbacks run on the main actor:

let observation = ValueObservation.tracking { ... }

let cancellable = observation.start(in: dbQueue { error in
    // Handle error on the main actor
} onChange: { value in
    // Handle fresh value on the main actor
}