ValueObservation Cleanup

[groue]

This pull request fixes some issues with ValueObservation in GRDB3:

• To guarantee asynchronous notifications (and never ever block your main thread), use the .async(onQueue:startImmediately:) scheduling:

  // On main queue
  var observation = ValueObservation.trackingAll(Player.all())
  observation.scheduling = .async(onQueue: .main, startImmediately: true)
  let observer = try observation.start(in: dbQueue) { (players: [Player]) in
      // On main queue
      print("fresh players: \(players)")s
  }
  // <- here "fresh players" is not printed yet.

  This scheduling used to be named .queue(_: startImmediately:) (a poor name which did not express the intent).

• ValueObservation.extent has been removed. Now all observations last until the observer returned by the start method is deallocated.