pg_upgrade: run all data type checks per connection

The checks for data type usage were each connecting to all databases
in the cluster and running their query. On clusters which have a lot
of databases this can become unnecessarily expensive. This moves the
checks to run in a single connection instead to minimize setup and
teardown overhead.

Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/BB4C76F-D416-4F9F-949E-DBE950D37787@yesql.se

Author: danielgustafsson