Bug 19790: Add a db revision for existing installs

Bug 19790: Add a db revision for existing installs

Since many installs may still have the additionalauthors kohafield,
this patch adds a dbrev in atomicupdate to clear it.

Test plan:
[1] Run updatedatabase. Check that you see no additionalauthors anymore
    in marc_subfield_structure.kohafield.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Charles Farmer <charles.farmer@inLibro.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>