Bug 31348: Make koha-plack stop gracefully

Bug 31348: Make koha-plack stop gracefully

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. Now at roughly the same time do the following two actions:
    3a. Go to http://localhost:8081/cgi-bin/koha/about.pl
    3b. koha-plack --stop kohadev
4. Note that the HTTP request completes
5. Click on "Home"
6. Note that the error is "Service Unavailable" rather than "Proxy Error",
which would've occurred with an interrupted HTTP request/response

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>