Namespace WorkRequest.task_name

In #340 (closed) we are introducing internal:* values for WorkRequest.task_name, which would otherwise contain lowercase class names as registered by BaseTask.

I propose to start namespacing BaseTask subclass names in WorkRequest.task_name with task:, while for compatibility assume that if a task_name does not contain a colon, a task:* prefix is assumed.

This should be a light and backwards-compatible change, that in the long term might provide cleaner handling and extensibility for that field: the moment we introduce namespaces with internal:, it seems like a good idea to me to plan to have all values namespaced in the long term