Provide a graceful way of stopping IJobWorker instances

[bobcat1506]

Describe the solution you'd like
Provide a graceful way of stopping IJobWorker instances such as a StopAsync() method.

[ChrisKujawa]

So instead of the Dispose method you would like to have a method which returns a Task? Currently the Dispose is kind of async it just cancels the CancellationTokenSource which then eventually will cancel the running threads.

[ChrisKujawa]

IAsyncDisposable is available for .net core 3 + https://docs.microsoft.com/de-de/dotnet/api/system.iasyncdisposable?view=net-5.0

As long we want to support core 2.1, we can't implement this interface.