Modify pg_stat_get_activity to build a tuplestore
Modify pg_stat_get_activity to build a tuplestore

This updates pg_stat_get_activity() to build a tuplestore for its
results instead of using the old-style multiple-call method.  This
simplifies the function, though that wasn't the primary motivation for
the change, which is that we may turn it into a helper function which
can filter the results (or not) much more easily.

lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,zeroae/postgres-xl,adam8157/gpdb,ovr/postgres-xl,yazun/postgres-xl,greenplum-db/gpdb,adam8157/gpdb,lisakowen/gpdb,adam8157/gpdb,oberstet/postgres-xl,ashwinstar/gpdb,yazun/postgres-xl,yazun/postgres-xl,jmcatamney/gpdb,lisakowen/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,Postgres-XL/Postgres-XL,ashwinstar/gpdb,50wu/gpdb,jmcatamney/gpdb,50wu/gpdb,50wu/gpdb,adam8157/gpdb,pavanvd/postgres-xl,ovr/postgres-xl,jmcatamney/gpdb,50wu/gpdb,pavanvd/postgres-xl,xinzweb/gpdb,xinzweb/gpdb,adam8157/gpdb,oberstet/postgres-xl,xinzweb/gpdb,techdragon/Postgres-XL,lisakowen/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,adam8157/gpdb,pavanvd/postgres-xl,Postgres-XL/Postgres-XL,lisakowen/gpdb,ovr/postgres-xl,ovr/postgres-xl,pavanvd/postgres-xl,xinzweb/gpdb,greenplum-db/gpdb,Postgres-XL/Postgres-XL,50wu/gpdb,jmcatamney/gpdb,adam8157/gpdb,lisakowen/gpdb,yazun/postgres-xl,50wu/gpdb,techdragon/Postgres-XL,oberstet/postgres-xl,50wu/gpdb,yazun/postgres-xl,Postgres-XL/Postgres-XL,lisakowen/gpdb,zeroae/postgres-xl,techdragon/Postgres-XL,pavanvd/postgres-xl,oberstet/postgres-xl,zeroae/postgres-xl,xinzweb/gpdb,zeroae/postgres-xl,techdragon/Postgres-XL,xinzweb/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,lisakowen/gpdb,xinzweb/gpdb,techdragon/Postgres-XL,greenplum-db/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,oberstet/postgres-xl,greenplum-db/gpdb,xinzweb/gpdb,Postgres-XL/Postgres-XL,jmcatamney/gpdb,ashwinstar/gpdb,ovr/postgres-xl,zeroae/postgres-xl,adam8157/gpdb