Add priorities to tasks/dependencies

[JanKoelzer]

We talked about adding priorities to tasks/dependencies to improve the performance of evaluations. NextCallbacks and CompleteCallbacks with a high priority should be called earlier.

The implementation in https://github.com/JanKoelzer/reactive-async/tree/jan-prio-ThreadPoolExecutor uses a ThreadPoolExecutor instead of a ForkJoinPool to execute tasks. This might influence performance. So benchmarks to test the overall performance are appreciated.

(Not that this is based on the PR on lazy computation.)

[JanKoelzer]

TODO: If #75 gets accepted, add priorities for sequential callbacks as well.