Revert "Fix an O(N^2) problem in foreign key references".
Revert "Fix an O(N^2) problem in foreign key references".

Commit 5ddc72887a012f6a8b85707ef27d85c274faf53d does not actually work
because it will happily blow away ri_constraint_cache entries that are
in active use in outer call levels.  In any case, it's a very ugly,
brute-force solution to the problem of limiting the cache size.
Revert until it can be redesigned.

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