Speed up Client#push_batches

* Make sure updated_at refers to the object's table name when pushing to Wombat

* Calling ```scope.send(filter.to_sym)``` loads all of the records into memory, so we spend a lot of time querying the same records every time #push_batches is called. Since we need to do the updated_at check anyway, it speeds things up to do it first and then call the filter.