Fix broken pre7->7 pg_upgrade related to the 64bit gxid work. (#11391)
Fix broken pre7->7 pg_upgrade related to the 64bit gxid work. (#11391)

There is no next gxid value in pg_control in pre-7 versions.  We need to
initialize it in those versions. Ideally during upgrade, those distributed logs
should be all zero-paged so there is no worry about distributed log format
change related to the 64bit gxid work. (I saw a TODO in
DistributedLog_Startup() but that should be thought separately).

Reviewed-by: David Krieger <360a392c008f9e97bd86c605fe1eb6e7ce44f9f0@pivotal.io>
Reviewed-by: Kalen Krempely <9e21ad9b1cb949be1035df9e3f38e74e41b67a0b@vmware.com>
50wu/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,adam8157/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,adam8157/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,adam8157/gpdb,lisakowen/gpdb,lisakowen/gpdb,xinzweb/gpdb,50wu/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,lisakowen/gpdb,lisakowen/gpdb,greenplum-db/gpdb,xinzweb/gpdb