storage format version number for external modules

Right now the backend server has a catalog version number which reports
the version of the system tables and storage format. It would be
helpful if pg_upgrade could access a storage format version number for
plugins like /contrib so it could check to see if the cluster can be
upgraded with the installed plugins. (A catalog version number is not
necessary for plugins.)

--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +