Allow custom script to handle bouncer stop

[blotus]

When the bouncer stops, the custom script does not know about it, preventing it from being able to clean up any resources it might have created.

We should:

• Add a shutdown message when invoking the script for each decision to allow it to clean anything up if it needs to
• Send SIGTERM instead of the default SIGKILL when feeding via stdin to allow the script to catch it and clean up.

In both cases, we also need a termination_timeout parameter to handle custom scripts that would take too long to clean up. After that, we would SIGKILL the custom script (to avoid the bouncer getting stuck on stop).