Add support for querying multiple UDF collections at once.

The client must wrap the JSON objects for UDFCs in an ["OR", ...]
combinator, as we can't query for matches on multiple UDFC types for the
same row.

One note, in order to add the tree joins to the query only when necessary, the
"udf:plot" and "udf:tree" SQL are both empty, and depend on "udf".
This introduces some extra whitespace characters in the query, but doesn't hurt
anything.
It's nice to see so much code code go away, especially complicated code. I can't quite grok how the need for this went away though.