bug 2527: avoid targeting of items on hold shelf

bug 2527: avoid targeting of items on hold shelf

SQL fix to properly ensure that if an item is on
the hold shelf, it will not be used for request
targeting, which applies only to items that are
not already on the hold shelf.

Prior to this fix, when checking out an item on the
hold shelf that fills patron A's hold request, it
was possible for that request to not be marked as
filled if another patron had an item-level request
on the item; the second patron's request was
incorrectly targeted by that item.

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>