Implement cancellation of indexing works simply by allowing the user to call cancel() on the returned future.

In the work processing thread, if the future is complete with a CancellationException, we'll just skip the corresponding work. If it's not, we'll send the work to the REST client and add an exception handler (.exceptionally(...) to the user-returned future: if the future is completed with a CancellationException, we'll cancel the HTTP request (is it possible?).