Clean up and simplify jsonb_concat code.
Clean up and simplify jsonb_concat code.

Some of this is made possible by commit
9b74f32cdbff8b9be47fc69164eae552050509ff which lets pushJsonbValue
handle binary Jsonb values, meaning that clients no longer have to, and
some is just doing things in simpler and more straightforward ways.

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