Allow transactions that don't write WAL to commit asynchronously.
Allow transactions that don't write WAL to commit asynchronously.

This case can arise if a transaction has written data, but only to
temporary tables.  Loss of the commit record in case of a crash won't
matter, because the temporary tables will be lost anyway.

Reviewed by Heikki Linnakangas and Simon Riggs.

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