Enable memcache automatic compression, starting at 20k and only if compression gain is greater than 20%.

Allows storage of larger objects (over 1mb in size uncompressed), such as huge LDAP schemas.
Should also improve cache efficiency (allows more stuff to be stored in same memory) and reduce network latency (less data transfer)

lib/util.php

@@ -1416,6 +1416,7 @@ function common_memcache()
             } else {
                 $cache->addServer($servers);
             }
+            $cache->setCompressThreshold(20000, 0.2);
         }
         return $cache;
     }