Add named tasks

[ahopkins]

This is a fairly simple PR that adds something that adds naming asyncio.Task. If you create a named task, it will also allow you to add it to a registry (if requested) for simple callback after.

app.add_task(dummy, name="dummy_task")
task = app.get_task("dummy_task")

This PR adds the following new attributes to application instances:

• app.get_task
• app.cancel_task
• app.purge_tasks
• app.shutdown_tasks
• app.tasks

In addition, on app.add_task it has a return value of the new task IF you are inside of the running loop.

TODO:

• Simple API to cleanup the registry from done tasks
• Some more tests around task registration
• Potentially an easy callable to cancel tasks: app.cancel_task("foo")
• Potentially also providing the task as a return value on add_task when created inside application (and not in global scope where it is added to delayed execution)
• Solve app.stop() does not run tasks to completion #2267

Closes #2301
Closes #2267

[codeclimate]

Code Climate has analyzed commit ebc4a7a and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (86% is the threshold).

This pull request will bring the total coverage in the repository to 86.8% (0.0% change).

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[ahopkins]

This pull request has been mentioned on Sanic Community Discussion. There might be relevant details there:

https://community.sanicframework.org/t/add-task-that-contains-an-infinite-loop-will-cause-an-error-when-stopping-the-server/948/4