Move current distributed transaction ID out of the per-subxact struct.
Move current distributed transaction ID out of the per-subxact struct.

Only a top-level transaction can have a distributed transaction ID, so
this seems more logical.

jmcatamney/gpdb,ashwinstar/gpdb,edespino/gpdb,xinzweb/gpdb,yuanzhao/gpdb,edespino/gpdb,edespino/gpdb,adam8157/gpdb,yuanzhao/gpdb,janebeckman/gpdb,xinzweb/gpdb,50wu/gpdb,yuanzhao/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,lisakowen/gpdb,50wu/gpdb,ashwinstar/gpdb,yuanzhao/gpdb,adam8157/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,Chibin/gpdb,janebeckman/gpdb,lisakowen/gpdb,janebeckman/gpdb,adam8157/gpdb,janebeckman/gpdb,lisakowen/gpdb,50wu/gpdb,jmcatamney/gpdb,Chibin/gpdb,edespino/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,Chibin/gpdb,yuanzhao/gpdb,janebeckman/gpdb,yuanzhao/gpdb,adam8157/gpdb,lisakowen/gpdb,greenplum-db/gpdb,edespino/gpdb,yuanzhao/gpdb,yuanzhao/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,50wu/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,Chibin/gpdb,Chibin/gpdb,edespino/gpdb,50wu/gpdb,janebeckman/gpdb,adam8157/gpdb,jmcatamney/gpdb,yuanzhao/gpdb,ashwinstar/gpdb,edespino/gpdb,greenplum-db/gpdb,xinzweb/gpdb,Chibin/gpdb,adam8157/gpdb,greenplum-db/gpdb,Chibin/gpdb,janebeckman/gpdb,yuanzhao/gpdb,adam8157/gpdb,greenplum-db/gpdb,lisakowen/gpdb,Chibin/gpdb,Chibin/gpdb,Chibin/gpdb,janebeckman/gpdb,xinzweb/gpdb,ashwinstar/gpdb,edespino/gpdb,50wu/gpdb,janebeckman/gpdb,edespino/gpdb,janebeckman/gpdb,edespino/gpdb,greenplum-db/gpdb