Bug 20047: Add Koha::Z3950Server Oject and use it to get server count

Bug 20047: Add Koha::Z3950Server Oject and use it to get server count

To test:
1 - prove t/db_dependent/Koha/Z3950Servers.t
2 - Load cataloging and authority home pages
3 - Verify you can add form z395 on each page
4 - Delete all z3950 servers
5 - Viery option to add from Z3950 is removed on both pages

Signed-off-by: Jesse Maseto <jesse@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>