… so that we can join to the correct table if not Tickets.  An
extensible registration system is introduced to determine how to join
CFs to applied objects.  The join used depends on the tuple of
(collection class, lookup type).  For example, consider that both
(RT::Ticket, RT::Queue-RT::Ticket) and (RT::Queue, RT::Queue) require no
additional join (so they return "main").

This lets _CustomFieldJoin handle joining for transaction CFs or other
CFs which may want to be searched.