Check if hold is really available in Trigger::Validator.

Check if hold is really available in Trigger::Validator.

Add checks for shelf time and no fulfillment time in
HoldIsAvailable in OpenILS::Application::Trigger::Validator.

We've had some issues with poorly migrated data and people
messing with holds that causes the original logic to trigger
for unfilled holds. Plus, the extra precaution of checking
more precise conditions of an actually filled hold can't hurt,
can it?

Signed-off-by: Jason Stephenson <jstephenson@mvlc.org>