Clear hold shelf/shelf_expire time for pickup lib change

Clear hold shelf/shelf_expire time for pickup lib change

If a hold is on the holds shelf and the pickup library changes, clear
the shelf_time and shelf_expire_time, since the hold is now in transit.

This is particularly important for avoiding action_trigger events for
the "hold_request.shelf_expires_soon" hook for what is now an
in-transit item.

Signed-off-by: Bill Erickson <berick@esilibrary.com>
Signed-off-by: Thomas Berezansky <tsbere@mvlc.org>