Finish implementation of hashed aggregation. Add enable_hashagg GUC

parameter to allow it to be forced off for comparison purposes.
Add ORDER BY clauses to a bunch of regression test queries that will
otherwise produce randomly-ordered output in the new regime.

src/backend/utils/misc/postgresql.conf.sample

@@ -83,6 +83,7 @@
 #enable_indexscan = true
 #enable_tidscan = true
 #enable_sort = true
+#enable_hashagg = true
 #enable_nestloop = true
 #enable_mergejoin = true
 #enable_hashjoin = true