Add support for aborting sub-tasks

[danlaine]

If we spawn a task (e.g. with spawn_ref), and that task spawns sub-tasks, and we drop/abort the parent task, the child sub-tasks aren't aborted. This can lead to unexpected behavior.

For example, a p2p::authenticated::Network spawns sub-tasks for the tracker, router, etc.. Aborting the network handle doesn't stop these tasks, meaning that for most intents and purposes the network is still running.

We should have a mechanism in the runtime to enable aborting of sub-tasks when the parent task is aborted.

[patrick-ogrady]

Related: #1104