Implement stack unwinding for cancelled tasks

[tsoutsman]

Depends on #766.

Closes #770.

Signed-off-by: Klim Tsoutsman [email protected]

[tsoutsman]

I've manually tested all three scenarios in test_task_cancel by rerunning test_task_cancel and checking log outputs.

Not sure about the naming of start_remote_unwinding.

[tsoutsman]

Also, as discussed before, this won't be able to unwind all tasks. Should cancel_task spawn another task that, after some timeout period (e.g. 5 seconds), checks if the original task has been unwound and, if not, aborts it?