Bug 21366: Add koha-plack --reload to gracefully restart Plack/Starman

Bug 21366: Add koha-plack --reload to gracefully restart Plack/Starman

This patch adds a "--reload" option to koha-plack, which allows
a sysadmin to gracefully restart Koha.

This is very useful when installing Koha plugins or deploying a hot fix
where you need to change code but don't want to interrupt anyone's
usage of Koha.

0. Apply patch
1. cp /usr/sbin/koha-plack /usr/sbin/koha-plack.bak
2. cp debian/scripts/koha-plack /usr/sbin/koha-plack
3. ps -efww | grep "starman"
4. Now at roughly the same time do the following two actions:
    4a. Go to http://localhost:8081/cgi-bin/koha/about.pl
    4b. koha-plack --reload kohadev
5. Note that the web request completes successfully
6. ps -efww | grep "starman"
7. Note that the "starman master" process stays alive, but the "starman worker" processes have been restarted

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>