Bug 26947: (follow-up) Set key checks to disabled for kohastructure

Bug 26947: (follow-up) Set key checks to disabled for kohastructure

This patch adds sql calls to disable foreign key checks for the initial
kohastructure schema load and then re-enables them again for subsequent
actions.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>