return task id when was action successful

This facilitates testing the service and is also a reasonable feature of
the API.
The API of TaskService#setDone() was also changed to make the testing
code easier: Instead of an int-typed task id it now takes a long-typed
task id.
tidy up tests: find tasks by id

Instead of getting the whole list of tasks and "hoping" that we found
the tasks we just added use the newly created method
TaskService#findById.
add happy path test for #findById
add TODOs for futher tests