Bug 14733: Prevent a record from having holds with duplicate priorities

Bug 14733: Prevent a record from having holds with duplicate priorities

It is possible to create holds with duplicate priorities.

The reason for this is that typically the priority is calculated before
placing the hold. When the hold is placed the priority is calculated.
This can easily be shown by opening up two browser windows and starting
to place a hold for a record in each one. You'll see that both list the
same priority. If you than place the hold in each window, both holds
will have the same priority!

Test Plan:
1) Run unit tests pre-patch, note they fail
2) Run unit tests post-patch, note they succeed

Signed-off-by: Heather Braum <hbraum@nekls.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>