Bug 33870: Make $MAX in Sitemapper::Writer global

Bug 33870: Make $MAX in Sitemapper::Writer global

This should preferably not be a lexical variable (in case we ever
should call this under Plack). A global variable will also simplify
testing (see next patch).

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