* [sales] remove availability check before adding to slot queue
* [sales] add missing return statement
* [tests] remove 'eventuallyCheck' helper
* [sales] remove reservations from slot queue
* [tests] rename module `eventually` -> `always`
* [sales] increase slot queue size
Because it will now also hold items for which we haven't
checked availability yet.
* Simplify `.then` (promise api) and tests
* Remove tracked future when cancelled. Add tracked future tests
* Track and cancel statemachine futures
The futures created in each asyncstatemachine instance are tracked, and each future is cancelled and waited in `stop`.
Change `asyncstatemachine.stop` to be async so `machine.trackedFutures.cancelAndWait` could be called.
Add a constructor for `asyncstatemachine` that initialises the `trackedFutures` instance, and call the constructor from derived class constructors.
markspanbroek
Eric