Re-adding a CAR file should keep allocations

[hsanjuan]

iirc, allocations when adding a file are decided up-front. Since there is no CID, we do not know if the item is pinned and therefore existing allocations are not re-used.

Now, with CAR files, we do have a root CID up-front, so we could at the very least send the blocks to the places that should already have them and not re-allocate the whole pin.