Greatly sped up pagination in the list builder.
1. The list builder builds a separate (optionally cached) paginator for each
page group, rather than each list. No more filling of the Tcl cache with (say) 5500
forum entries when we know that users rarely march through the entire list of forums.
2. Since the list builder builds an "in (list of ids)" query clause to select
just those rows needed for this display page, there's no reason to sort the list
of ids so I'm building the list within the list builder rather than using
the more generalized paginator code. Minor speedup ...
This could probably be sped up more by simply caching the list of keys within
the list builder and replicating some other paginator functionality in lightweight
form specific to the list builder, but this will do for now. And maybe forever.