Contrib modules pgrowlocks, pgstattuple and some functionality in
pageinspect currently only supports the heap table AM. As they are all
concerned with low-level details that aren't reasonably exposed via
tableam, error out if invoked on a non heap relation.

Author: Andres Freund
Discussion: https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de