Bug 17736: [Follow-up] Rename to current_holds

Bug 17736: [Follow-up] Rename to current_holds

It is not about when the hold was 'placed' but if the hold pertains to
the future or not.

Test plan:
[1] Git grep on holds_placed_before_today.
[2] Run t/db_dependent/Koha/Biblios.t
[3] Run t/db_dependent/Reserves.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>