Bug 27584: Refactor OAI-PMH paging to improve performance

Bug 27584: Refactor OAI-PMH paging to improve performance

Includes the following optimizations:
- Use next biblionumber instead of large offset in the queries.
- Use unions instead of subqueries
- Avoid fetching item timestamps when items are not included.

Test plan:

1. Without the patch, try harvesting a Koha database with (and without for good measure) `include_items: 1` in the OAI-PMH configuration file pointed to by preference OAI-PMH:ConfFile and take note of performance. For useful metrics the database must be large enough to not fit in InnoDB buffers or OS file cache.
2. Apply the patch.
3. Run tests: prove -v t/db_dependent/OAI
4. Try again the harvesting from step 1 and compare performance with step 1.

Signed-off-by: David Cook <dcook@prosentient.com.au>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>