Add timeouts in case psql doesn't deliver the expected output, and try
to cause the monitoring psql to be fully connected to a backend.  This
isn't necessarily everything needed, but at least the timeouts should
reduce the pain for buildfarm owners.

Author: Andres Freund
Reported-By: Tom Lane, BF animals prairiedog and calliphoridae
Discussion: https://postgr.es/m/E1du6ZT-00043I-91@gemulon.postgresql.org