Filter destructors called upon daemon termination MP-505

When metaproxy daemon receives SIGTERM the stop method of each filter
is called. The stop method existed before, but did not take any
arguements (void).. The frontend_net filter can now distinguish
between SIGUSR1 (stop bind, but keep sessions running) and SIGTERM
(stop immediately). Note that this changes the API+ABI of Metaproxy
filters.