Implement two step interruption

[Flood1993]

Implement two step interruption for agents/name server. First SIGINT captured results in clean exit/kill (i.e.: agent waits for code execution and tear-down and name server waits for all agents to be unregistererd). If another SIGINT is captured, kill with no mercy (as we are doing it right now).

[Flood1993]

In the following branch https://github.com/Flood1993/osbrain/tree/sigint_shutdown I have been doing some research.

Original commit message for future reference:

    This will have to be rebased so as to clean the mess.
    
    In this commit are the settings which I have been using to try a
    clean shutdown with Ctrl + C. The idea behind it is, launch
    `python test.py` from a console and then kill it with Ctrl + C.
    
    We can see that the daemon is the first one who gets the signal,
    and right now it prevents the processes from getting it. By
    reverting the changes in nameserver.py:92, we can see that the
    problem is that we are not able to locate the NS after Ctrl + C,
    so the NameServerProcess cannot know when all agents have been
    successfully shutdown.
    
    Further investigation is required.```