to uniquely identify a record from its PID, Invenio requires the tuple
PID type and value, not only the value. Changed the item_pid field to
be an object type and value.
changed methods that were expecting item_pid as a string to now expect
an object instead
removed unused view circulation/items to retrieve the item from the
loan
added loan parameter to $ref builder functions to allow more
flexibility when implementing the method
added transaction location and user validations on REST loaders
added schema loader to validate REST request when replacing the item
attached to the loan