Add vacuum_freeze_table_age GUC option, to control when VACUUM should

doc/src/sgml/catalogs.sgml

doc/src/sgml/config.sgml

doc/src/sgml/maintenance.sgml

src/backend/commands/cluster.c

src/backend/commands/vacuum.c

src/backend/commands/vacuumlazy.c

src/backend/nodes/copyfuncs.c

src/backend/nodes/equalfuncs.c

src/backend/parser/gram.y

src/backend/postmaster/autovacuum.c

src/backend/utils/misc/guc.c

src/backend/utils/misc/postgresql.conf.sample

src/include/catalog/catversion.h

src/include/catalog/pg_autovacuum.h

src/include/commands/vacuum.h

src/include/nodes/parsenodes.h