Add upper bound on postgresql-libpq

I have a desire to remove the large object bindings from postgresql-libpq.
This will probably happen with version 0.10,  though I have no particular
timeframe in mind yet.  Equivalent large object bindings in terms of exec
(or their async counterparts) will need to be written.   I'm not sure
where these should be placed;  perhaps in postgresql-libpq,  perhaps in
-simple,  or another package altogether.

One advantage to this approach is that we can support 64-bit large objects
without having to worry about bumping the minimum dependency on libpq proper.