Bug 23517: Add the controller method for holds priorities handling

Bug 23517: Add the controller method for holds priorities handling

This patch implements the PUT /holds/:hold_id/priority endpoint
To test:
- Apply this patches
- Run:
  $ kshell
 k$ prove t/db_dependent/api/v1/holds.t
=> SUCCESS: Tests pass!
- Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Liz Rea <wizzyrea@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>