Improve PQL parse performance with high number of filter clauses

We've discovered that PQL parsing (via instaparse) can be extremely slow (say 20s for a pql query with ~1000 filter clauses). A brief investigation suggests that this may be a known issue with instaparse.

It's not yet clear whether we might be able to address this by rearranging our grammar somehow, or we might want to rewrite the parser (either via manual recursive descent, or via something like antlr, flex/bison, etc.