Serve RPC from CAR, no local blockstore

[rvagg]

Load a CAR as a blockstore and use it read-only. go-car can already do this, with CARv1 it'll have to make an internal index for it but if you have a CARv2 it can load an existing index if there is one.

This requires #12846 be implemented so Lotus isn't doing a sync and generating new blocks.

Challenges that will come up doing this (that I can think of, there will be more):

• What do we do about the datastore? One critical thing it's used for right now is to set initial ChainStore state, most importantly the current tipset and also the genesis. Those two pieces are possible to hack around just by manually inserting them into the ChainStore, but there's likely other metadata in the datastore that wants to be loaded.
• Are there any surprising places where lotus will want to write blocks? Replaying state, estimating gas for messages? Those cases where you're not doing real state work should be handled by using a layered blockstore and doing the temporary stuff in memory but we are doing that consistently? What about events?
• What about other on-disk items, can we do without them, do we make a tmp directory for them, do we do them in memory?