Explicit ValueObservation cancellation

[groue]

ValueObservation used to be stopped when the result of the start method would get deinitialized.

Now you can also call the cancel() method explicitly:

// BEFORE: GRDB 4
let observer: TransactionObserver?
observer = observation.start(...)
observer = nil       // Stop the observation

// NEW: GRDB 5
let cancellable: DatabaseCancellable
cancellable = observation.start(...)
cancellable.cancel() // Stop the observation

The returned DatabaseCancellable cancels itself when it gets deinitialized.