Lookup attributes in the cache, or retrieve them from the object if necessary.
If this is a ObjectCache then this will lookup the attributes there first if available, otherwise will read them from the object and update the cache.
If this is not a ObjectCache, then the attributes will simply be read from the object.
This will return immediately and complete asynchronously
| this |
the object |
| attr_types |
the types of attributes to update |
| cancellable |
optional cancellation object |
| callback |
called when the operation completes |
| n_attr_types |
the number of attribute types |
| user_data |
data to pass to the callback |