Make deserializer from cache backward compatible

[niobos]

The (de)serialization currently uses Pickle. This breaks when upgrading the classes of the stored objects, since the deserializer can't find the newly added fields in the file.

We need a backward compatible deserialization process. This probably means writing a (de)serializer explicitly instead of relying on pickle. (This has an added security advantage; unpickle-ing isn't very safe to do if the source is not trusted)

See also: #69