Bug 19725: OAI-PMH - Use biblio_metadata.timestamp

Bug 19725: OAI-PMH - Use biblio_metadata.timestamp

Since bug 17196, biblioitems.timestamp is not always updated after a
change in the MARC record.
Filtering should be based on biblio_metadata.timestamp instead.

Test plan:
1. prove t/db_dependent/OAI/Server.t
2. Verify that it SUCCEEDS

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>