Bug 13690: use Koha::Schema only when it's needed

Bug 13690: use Koha::Schema only when it's needed

This patch makes Koha::Database lazy-load the whole Koha::Schema libraries.
It doesn't seem to have negative effects, and makes scripts not using
DBIx::Class notably faster [1].

Regards

[1] If you read the DBIx::Class::Schema docs, it explains that it it uses
Module::Find to load all schema files...
http://search.cpan.org/~ribasushi/DBIx-Class-0.082810/lib/DBIx/Class/Schema.pm#load_namespaces

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
Happy to sign this one. The only extra comment would be that DBIx::Class
performance issues remain after this patch, but is being handled in a
different bug.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>