Add stop behaviour to GenServer

[juan518munoz]

Add a CancellationToken to our Actors.

Idea behind is that we can now stop all the running services working around an actor (timers and listeners as of now) when requested, so they are not left hanging.

Note: this PR has no changes to the threads impl due to being no longer maintained, and should only be merged after stream listener PR.

Closes #18

[ElFantasma]

Ah, nice! I like having both, the individual cancellation_token, and the gen_server associated one

[ElFantasma]

Good work!
I think the cancellation token logic is missing for stream_listener, maybe?

Also, I think that teardown logic should be a more complete workflow:

• teardown() should not be called explicitly on the handler (as it is now).
• Instead, teardown() should be called automatically on exiting the GenServer main_loop, either because of an error, or because of normal termination (ie. returning a Stop outbound message on a callback), probably here and here
• It should in turn call a teardown() function in the GenServer (giving the opportunity to reimplement it as a callback, similar to init() in Trait implementations, to gracefully shutdown it, as well as liberating any resource potentially being held).

Does it make sense?