add async filter optimisation for cast removal

[chris-twiner]

given

create view table_view select *. cast( longfield as string) longfield_string from thetable;
select * from table_view where longfield_string = '12353463434'

the query will be treated as:

select * form table_view where cast (longfield as string) = '12353463434'

it should instead be treated as:

select * form table_view where longfield = cast ( '12353463434' as long) 

which will constant fold and allow longfield to be pushed down

[chris-twiner]

low prio due to demand change and potentially not sufficient configuration concept (i.e. given how much cast does it's possible a case is not possible to unpack - non-deterministic expressions etc.)