Enhancement: Add Tasks API endpoint to return state of the task.

Because we're already tracking the state of the tasks using events, we should expose it via the API. We've got a bunch of tasks that we spawn with `ignore_result=True` and hence `AsyncResult` always returns `PENDING` for them, and so does the `/api/task/result/` endpoint, however, internally, we're tracking the state of the task, and we can expose it if anyone is interested. 

Willing to do any more work required to get this in. I think at least a few people will find it helpful. :)