Make max_parallel_degree PGC_USERSET.
Make max_parallel_degree PGC_USERSET.

It was intended to be this way all along, just like other planner
GUCs such as work_mem.  But I goofed.

50wu/gpdb,jmcatamney/gpdb,xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,50wu/gpdb,ashwinstar/gpdb,adam8157/gpdb,adam8157/gpdb,lisakowen/gpdb,adam8157/gpdb,lisakowen/gpdb,jmcatamney/gpdb,xinzweb/gpdb,50wu/gpdb,50wu/gpdb,adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,ashwinstar/gpdb,lisakowen/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,50wu/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,xinzweb/gpdb,greenplum-db/gpdb,adam8157/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,50wu/gpdb,adam8157/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,jmcatamney/gpdb,lisakowen/gpdb,jmcatamney/gpdb,xinzweb/gpdb,lisakowen/gpdb,jmcatamney/gpdb,lisakowen/gpdb