Expose Disposable responses from network tasks

[stefan-niedermann]

No practical benefit yet.

The idea is to get more control over the background tasks in the long term (maybe the ability to cancel a running sync?) instead of just shooting into the dark like "fire and forget". (That's why i also used ExecutorService where possible, to avoid starting threads without control over it)

Drawbacks: Deep dependency to RxJava2 all over the app - not sure whether it's worth it.

ToDo:

• Catch new CancellationException and handle it on the UI (probably by just doing nothing)