Resdhift's lack of cursors and PQsetSingleRowMode

Hi:

We're using psycopg2 to connect to Amazon's Redshift, and one problem
Redshift has is that it lacks server-side cursors. What I'd like is to
be able to use PQsetSingleRowMode to avoid consuming all the data at
once (in specific circumstances, of course). I'm not entirely sure what
in psycopg2 would need to be modified, but it seems like the changes
would mostly be restricted to the `async == 0` portion of `pq_execute`,
but I'm not familiar enough with the source to be certain.

-Ryan Kelly