Allow numeric to use a more compact, 2-byte header in many cases.
Allow numeric to use a more compact, 2-byte header in many cases.

Review by Brendan Jurd and Tom Lane.

oberstet/postgres-xl,ashwinstar/gpdb,postmind-net/postgres-xl,tpostgres-projects/tPostgres,ashwinstar/gpdb,techdragon/Postgres-XL,snaga/postgres-xl,techdragon/Postgres-XL,jmcatamney/gpdb,kmjungersen/PostgresXL,snaga/postgres-xl,xinzweb/gpdb,greenplum-db/gpdb,ovr/postgres-xl,oberstet/postgres-xl,xinzweb/gpdb,yazun/postgres-xl,lisakowen/gpdb,ovr/postgres-xl,50wu/gpdb,50wu/gpdb,adam8157/gpdb,snaga/postgres-xl,xinzweb/gpdb,xinzweb/gpdb,jmcatamney/gpdb,yazun/postgres-xl,techdragon/Postgres-XL,techdragon/Postgres-XL,oberstet/postgres-xl,ovr/postgres-xl,adam8157/gpdb,pavanvd/postgres-xl,pavanvd/postgres-xl,greenplum-db/gpdb,pavanvd/postgres-xl,zeroae/postgres-xl,kmjungersen/PostgresXL,greenplum-db/gpdb,lisakowen/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,techdragon/Postgres-XL,greenplum-db/gpdb,zeroae/postgres-xl,adam8157/gpdb,oberstet/postgres-xl,Postgres-XL/Postgres-XL,Postgres-XL/Postgres-XL,arcivanov/postgres-xl,tpostgres-projects/tPostgres,jmcatamney/gpdb,greenplum-db/gpdb,postmind-net/postgres-xl,adam8157/gpdb,kmjungersen/PostgresXL,xinzweb/gpdb,kmjungersen/PostgresXL,ashwinstar/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,yazun/postgres-xl,arcivanov/postgres-xl,greenplum-db/gpdb,adam8157/gpdb,pavanvd/postgres-xl,pavanvd/postgres-xl,adam8157/gpdb,ovr/postgres-xl,postmind-net/postgres-xl,adam8157/gpdb,xinzweb/gpdb,zeroae/postgres-xl,50wu/gpdb,jmcatamney/gpdb,kmjungersen/PostgresXL,50wu/gpdb,Postgres-XL/Postgres-XL,arcivanov/postgres-xl,postmind-net/postgres-xl,oberstet/postgres-xl,zeroae/postgres-xl,ovr/postgres-xl,zeroae/postgres-xl,50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,lisakowen/gpdb,yazun/postgres-xl,lisakowen/gpdb,xinzweb/gpdb,ashwinstar/gpdb,xinzweb/gpdb,jmcatamney/gpdb,snaga/postgres-xl,arcivanov/postgres-xl,tpostgres-projects/tPostgres,Postgres-XL/Postgres-XL,ashwinstar/gpdb,50wu/gpdb,ashwinstar/gpdb,tpostgres-projects/tPostgres,arcivanov/postgres-xl,tpostgres-projects/tPostgres,lisakowen/gpdb,Postgres-XL/Postgres-XL,postmind-net/postgres-xl,lisakowen/gpdb,adam8157/gpdb,yazun/postgres-xl,lisakowen/gpdb,lisakowen/gpdb,snaga/postgres-xl,arcivanov/postgres-xl,50wu/gpdb,ashwinstar/gpdb