[brisbane-core/MERGE] inventory fix & refactor

[Vincent Ladeuil]

<snip/>

    jam> I may be one of the only/few that feel this way, so I'm
    jam> happy to start a discussion on it.

I strongly feel that way too (I sent my email just before reading
yours :), I'm happy to discuss the subject too.

May be fast-import is an exception that needs to maintain a
mutable inventory while still being able to create CHKInventory
objects as fast as possible, but even in that case, there should
be a way to make that efficient without requiring CHKInventory to
become mutable.

       Vincent