engine/postgres: Upgrade to pg_query_go/v4

[kyleconroy]

It appears the v4 does not support ? placeholders anymore. The following query fails with a parse error:

SELECT * FROM users WHERE id = ?;

Update: These queries would never have worked in the first place, as the PostgreSQL server does not support the ? syntax. Going to go ahead and merge this :)

[oliverpool]

I have been using the postgres engine and then running my code on sqlite (since the sqlite engine is missing a few features).
So I just got bitten by this change (but since my workflow is hacky, this is not a real surprise).

Replacing all ? is not so tedious and thanks to compile time type checking, I am pretty confident of update 👍