Bug 21015: fix performance issue with C4::Members

Bug 21015: fix performance issue with C4::Members

loading Koha::Schema ("use Koha::Schema;") takes significantly time as
it loads almost a couple hundred classes. Koha::Database has done that
already once and we can use it to get the ResultSet "Borrower" as
well, so let's use that. This also make the code more unified because
Koha::Database is used throughout the code instead of Koha::Schema.

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: Nick Clemens <nick@bywatersolutions.com>