It turns out that there are obscure cases where this matters, so we can't
as easily just ignore identity CIDs. Specifically, classic online Filecoin
deals that rely on Graphsync _and_ require identity CIDs be stored in the
CARv1 that is used to calculate CommP must see the identity CID pass through
the LinkSystem.

Unfortunately, the easiest way to deal with this is to send them over the wire
as if they are a normal block; which happens to be the "safe" backward
compatible way too. Less easy way would be to simulate it on both ends and
not send them, but we'll take the easy path for now.

Extension of tests in here to make sure that the full DAG is transferred even
in this case. Blockstore _must_ have identity CIDs in them, or be able to
respond properly to them.