Hold current_shelf_lib DB and IDL

Hold current_shelf_lib DB and IDL

Schema, upgrade, and IDL pieces for a new
action.hold_request.current_shelf_lib column.  The goal of the column is
to track the location where a hold is currently sitting on a hold shelf.
Typically, this will be the pickup library, however, if the pickup
library has changed while the hold was on the shelf, the current shelf
lib will be different.

The upgrade script sets current_shelf_lib to match the pickup_lib for
all holds that are currently active and on the shelf.

Signed-off-by: Bill Erickson <berick@esilibrary.com>