Cell resolution depends on order of CellCompleters

[JanKoelzer]

resolveDefault and resolveCycle use the Key of the first cell of the HandlerPool. Therefore the order of the cells matters, which is unintuitive. A test "whenNext: Cycle resolution, order of CellCompleters" can be found here: https://github.com/JanKoelzer/reactive-async/tree/jan-tests-cycles