Limit the optimizer mdcache default size to 16MB.

Based on the experiments with tpch and tpcds, we found that 16MB would
be the optimal size of mdcache.
[#96490982].

src/backend/utils/misc/guc_gp.c

@@ -4516,7 +4516,7 @@ struct config_int ConfigureNamesInt_gp[] =
 			GUC_UNIT_KB | GUC_GPDB_ADDOPT
 		},
 		&optimizer_mdcache_size,
-		0, 0, INT_MAX, NULL, NULL
+		16384, 0, INT_MAX, NULL, NULL
 	},
 
 	{