watchdog: fix another possible watchdog timeout

[alexmohr]

dlt_daemon_process_user_message_log prior to this
commit was running without limit, meaning when a lot of messages have to be transmitted dlt-daemon might
be killed by systemd watchdog due to being busy
with sending and not updating the watchdog anymore. This commit introduces a timeout in dlt_daemon_process_user_message_log which allows this function to run at most watchdog_trigger_interval seconds.

This MR extends the logic introduced in #595 as the original commit still allowed systemd to kill dlt-daemon

To test we need the following things

• dlt-daemon started via systemd with enabled watchdog
• a lot of logging going into the daemon

Excecute the script below on your test system.
It spawns a lot of dlt-receive proceses, the goal is for dlt-daemon to survive.

end_time=$((SECONDS+180))

while [ $SECONDS -lt $end_time ]; do 
  dlt-receive -a 127.1 &
done 

killall -9 dlt-receive

if [[ $(uname) == "Linux" ]]; then
  systemctl status dlt.service
  echo Service should be running for > 3 minutes
else
  pidin -faAt | grep dlt-daemon | grep -v grep
  echo Service should be running at least since system boot
fi

The program was tested solely for our own use cases, which might differ from yours.
Licensed under Mozilla Public License Version 2.0

^{Alexander Mohr, alexander.m.mohr@mercedes-benz.com, Mercedes-Benz Tech Innovation GmbH, imprint}

[minminlittleshrimp]

imho, I am not sure about the fix.
Honestly I have not touched Watchdog feature in dlt yet 😁
I would love to spend sometime to check from my local some functionality of this option.
Thank you for the patch!