Don't spew warnings on usage of MEMCACHE_COMPRESSED constant when memcache PHP extension is not present.

Switched to a locally-defined Cache::COMPRESSED, translating that to MEMCACHE_COMPRESSED in the plugin.

classes/Memcached_DataObject.php

@@ -363,7 +363,7 @@ class Memcached_DataObject extends DB_DataObject
             $cached[] = clone($inst);
         }
         $inst->free();
-        $c->set($ckey, $cached, MEMCACHE_COMPRESSED, $expiry);
+        $c->set($ckey, $cached, Cache::COMPRESSED, $expiry);
         return new ArrayWrapper($cached);
     }