Allow to gracefully abort the tasks

[Baptouuuu]

Problem

If a source receive an external event telling it to stop running it can stop itself but the loop will wait for all tasks to finish.

Currently the only way for a developer to abort the tasks is to develop a custom way to send the information.

Possible solution

Add a Continuation::abort() method that would emulate a SIGTERM on each tasks through the OperatingSystem abstraction (as a parent process would do on its children)