Add a new lookaside module
    
    At the moment this doesn't do much: it only stores the 3
    lookaside-related parameters we have (a download url, an upload url and
    a hash type), but eventually it will become a full-fledged lookaside
    cache client.
    
    Using a property for it makes it trivial for downstreams to override it
    if they need to use their own implementation of a lookaside cache
    client.
    
    And using a cached property means we won't reinitialize the
    CGILookasideCache object every time we access the
    Commands.lookasidecache attribute.