only traverse cbor blocks when running mst diff (#425)

I spaced the fact that we include proper dag-cbor cid links to images
which are not actually included in the car slices. In the case where
someone drops a reference to an image (deletes an image post, or updates
their profile picture) then we would hit this 'ipld not found' error on
the image cid (since we never got it in the first place).

Here, we skip any CID that isnt CBOR, This is a reasonable assumption to
make for now, unless we start dangling random cbor graphs off of the
MST, at which point ill probably need to get more clever (I'd really
rather not).