Cache fixes:

Cache fixes:
* We now cache negative lookups; clear them in Memcached_DataObject->insert()
* Mark file.url as a unique key in statusnet.ini so its negative lookups are cleared properly (first save of a notice with a new URL was failing due to double-insert)
* Now using serialization for default in-process cache instead of just saving objects; avoids potential corruption if you save an object to cache, change the original object, then fetch the same key from cache again