fix(ns-plug): avoid mwan3 alerts during restart

[filippocarletti]

Running /etc/init.d/mwan3 restart produces a rapid disconnect/connect sequence which triggers an alert lasting a few seconds. This PR avoid the false alarm delaying disconnect alerts by 30 seconds.

#992 required.

See #1054

[filippocarletti]

The only issue remaining is the following log:

mwan3-hook: Script /usr/libexec/mwan-hooks/send-mwan-alert failed with exit code 129

Exit code 129 means that the process has been killed with SIGHUP (128 + 1).
We could handle 129 as a special case in /usr/libexec/ns-plug/mwan-hooks

[gsanchietti]

I really do not like this approach: it feels too hacky

Feel free to merge if your tests were good enough.
Before merge, please open an issue for it

[Tbaile]

If this is a disconnection, shouldn't be treated the same as the others? (the IT might want to be notified if at 10:30 AM the VOIP active connections dropped, otherwise this info is lost)

[Tbaile]

Talked /w Giacomo, while we think the change is fine, there's the need to open two PRs for this changes, one for the timeout change and the other for the alert during restart, so the work can be tracked easily (and reverted in even easier manner)

Ping me internally if you want me to do it