Fix up parallel-safety marking for appendrels.
Fix up parallel-safety marking for appendrels.

The previous coding assumed that the value derived by
set_rel_consider_parallel() for an appendrel parent would be accurate for
all the appendrel's children; but this is not so, for example because one
child might scan a temp table.  Instead, apply set_rel_consider_parallel()
to each child rel as well as the parent, and then take the AND of the
results as controlling parallel safety for the appendrel as a whole.

(We might someday be able to deal more intelligently than this with cases
in which some of the childrels are parallel-safe and others not, but that's
for later.)

Robert Haas and Tom Lane

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