Bug 30897: Gracefully restart plack after plugin install/remove

Bug 30897: Gracefully restart plack after plugin install/remove

This patch adds a graceful plack restart to the plugin
install/uninstall routine to fully allow plugin install to work via the
staff client.

Test plan
1. list your worker processes with something like htop
2. install a plugin
   https://github.com/bywatersolutions/dev-koha-plugin-kitchen-sink/releases
3. note that the PIDs of the workers stay the same and no sign of
   restart (no PID changes and no CPU activity that the workers would
   have if they were starting)
4. apply patches and restart services
5. unistall plugin
6. notice worker restart
7. install plugin
8. notice worker restart
9. downgrade plugin
10. notice worker restart
11. upgrade plugin
12. notice worker restart
13. disable and enable plugin
14. *no restart*

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>