Add a notion of a 'catalog version number' that can indicate

when an initdb-forcing change has been applied within a development cycle.
PG_VERSION serves this purpose for official releases, but we can't bump
the PG_VERSION number every time we make a change to the catalogs during
development.  Instead, increase the catalog version number to warn other
developers that you've made an incompatible change.  See my mail to
pghackers for more info.