[ASFXNB859] In FilterQueue, suppress duplicates

Huge efficiency change.

Until now, when we did a model::find() call, we added model::_find()
as a priority-0 callback. Then we ran the positive-priority (pre-query)
filters, then the primary query, then the negative-priority (post-query)
filters.

But we added the primary priority-0 callback to the queue on *every*
ifind() call. So a model instance that calls find() multiple times
in a single process - like the web/show-list worker - actually ends
up doing the primary query (and formatting the results) multiple times.

As a test, I added a counter to the find() method that counts how many
times the primary is called, and then echoes the value at the end of
the method. Then I ran the web/show-list worker, which performs lots
of finds on a single model instance. The count rose to 87.

Zoinks!

This change fixes this by suppressing attempts to add a filter
under a name if it has alreasy been added.

Wow, what a difference. :D