Add engine condition pushdown

Add engine condition pushdown to efficiently deal with restricting clauses like ...weight <= N
This can abort a search on a particular path, rather than returning too much and letting the server filter.

It's going to be a bit quirky, because while in the resultset each row only contains the weight of that one step, the weight restriction would operate on an entire path... it'll still satisfy the condition though, so I guess it's ok.