Rearrange Memcached_DataObject::staticGet() to avoid "only variables can be passed...

Rearrange Memcached_DataObject::staticGet() to avoid "only variables can be passed by reference" warnings when DB lookup fails and we return false.
(We need to keep it returning a reference because the extlib parent class is stuck in PHP 4-land and uses references everywhere, including this function's return value. Yuck!)

Also changed pkeyGet to drop the reference, since it doesn't have an upstream equivalent.