Denorm flush cached

I did it for myself, because I didn't want to update N-times the same objects. Additionaly I've added chunk parameter (defaults to 0=all).

    Additionaly this flush uses cache for each content_type and object_id.
    We calculate each content_object once - even we modified it N-times
    before last flush.
    Setting chunk to something greater than 0 will limit DirtyInstances
    selected in one pass.

PS: If You like it I can also add tests/docs ect. Just see if You find it useful.