Should we remove default timeout in shutdown?

[Peque]

Following the "explicit is better than implicit" mantra maybe we should remove the default value of 10., which is pretty arbitrary.

Maybe the shutdown call should wait forever unless a timeout is provided.

Maybe we should split the current shutdown-related methods into:

• .shutdown(): 100% clean shutdown, optional timeout which raises an exception
• .kill(): 0% clean shutdown, optional timeout which raises an exception
• .terminate(): similar to what we currently have, which tries to shutdown for a while and if that does not succeed, it tries to kill; timeout is not optional (and will raise an exception if reached)

Properly implemented agents should be able to .shutdown() cleanly and maybe the most intuitive behavior is for the system to wait until that happens (even if it takes longer than 10 seconds or any other arbitrary timeout).