Bug 11891 - Make Koha::Schema use C4::Context->dbh

Bug 11891 - Make Koha::Schema use C4::Context->dbh

Right now there is no connection between the database handles used by
C4::Context::dbh and Koha::Datbase/Schema. This makes it impossible to
use transactions in unit tests to temporarily modify the database to
test subroutines that take advantage of Koha::Database. This patch fixes
that issue.

Test Plan:
1) Apply this patch
2) prove -v t/db_dependent/ILSDI_Services.t and
   prove -v t/db_dependent/Items.t and
   prove -v t/db_dependent/Circulation_issue.t should
   all start passing

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>