When the killmode is 'process', only the mgt process is ever signalled
by systemd. If that fails to exit within TimeoutStopSec, we may leave
unresponsive processes till tearing themselves down. During a restart of
the service, that could lead to the old cache process still holding onto
resources like bound ports, preventing the new service from starting.

With 'mixed' mode instead, first SIGTERM is sent to the mgt process only
(found through its PID file), but all the processes in the group will be
sent SIGKILL after TimeoutStopSec, and systemd will wait for them all to
exit.
Signed-off-by: Dridi Boukelmoune <dridi.boukelmoune@gmail.com>