Follow-up from "Restrict creation of tasks (Closes: #348)"

The following discussion from !732 (merged) should be addressed:

• @enrico started a discussion: (+3 comments)

  An option that was discussed with @cjwatson was to index subclasses by task_type and then class name, so that there can be, say, a "noop" workflow and a "noop" task without conflicts.

  This would add a mandatory task_type argument to methods like is_valid_task_name and class_from_name, which in turn would mean extending the client/server API to pass task_type alongside task_name in WorkRequestRequest and WorkRequestResponse, unless we're restricting the client/server API to only create tasks for external workers (which would sound fair)