Support for Combine

[groue]

This PR provides the necessary tooling for the new companion library GRDBCombine, which extends GRDB with tools dedicated to the Combine framework.

• The ValueObservation.trackingCount/All/One methods are deprecated. Now please use instead the observationForCount/All/First request methods:

  -let observation1 = ValueObservation.trackingAll(Player.all())
  -let observation2 = ValueObservation.trackingOne(Player.filter(key: 42))
  +let observation1 = Player.observationForAll()
  +let observation2 = Player.filter(key: 42).observationForFirst()

• The ValueObservation.start(in:onError:onChange:) method used to always access the database synchronously - despite some documentation claims. It now no longer does, when the observation is configured for asynchronous dispatching. This makes sure that starting an observation does not block your current thread if a long-running write transaction is running in a background thread:

  // Current queue is the main queue.
  //
  // Unchanged: an observation configured for synchronous notification
  // of its initial value is accessing the database right on start.
  let observation1 = Player.observationForAll()
  observation1.start(
      in: dbQueue,
      onError: { ... },
      onChange: { (players: [Player]) in print("fresh players: \(players)") })
  // Unchanged: here "fresh players" is guaranteed to be printed.
  
  // NEW: An observation configured for asynchronous dispatch
  // of its initial value is no longer accessing the database right on start:
  var observation2 = Player.observationForAll()
  observation2.scheduling = .async(onQueue: .main, startImmediately: true)
  observation2.start(
      in: dbQueue,
      onError: { ... },
      onChange: { (players: [Player]) in print("fresh players: \(players)") })
  // Unchanged: here "fresh players" is not printed. It will, eventually.

• The ValueObservation.start(in:onError:onChange:) method is no longer throwing when you provide an error callback. All database errors are then guaranteed to be sent to this callback, even synchronous startup errors.

• The ValueObservation.combine method has been enhanced:

  // NEW: up to eight observations
  let observation = ValueObservation.combine(obs1, obs2, ..., obs8)
  // NEW: combine instance method:
  let observation = obs1.combine(obs2) { value1, value2 in ... }

• The ValueObservation.distinctUntilChanged() method has been deprecated in favor of removeDuplicates(). Yes, yes, GRDB will follow the naming conventions of Combine when they do not match ReactiveX.