Initial attempt at optimizing has_child query.

Short-circult bitset iterations once all parents have been found.
Add more has_child tests.
Add more has_child filter benchmarks
Minor optimization to has_child filter

Short-circuit id lookups once all parents have been found and don't
perform any lookup when no parents are found.
Don't execute children filter in has_parent filter when no parents have been found.
Don't execute child filter when no parents have been found in parent query.