Serve limited RPC from test snapshot CAR

[rvagg]

From Forest: https://docs.forest.chainsafe.io/developers/guides/rpc_test_snapshot/

If we can serve RPC from a CAR, we should also be able to do it from a minimal CAR that just has the blocks needed to serve a specific path. This might be tricky because it assumes that Forest and Lotus will require the same blocks to serve a call but it may be the case Lotus does more work to get some calls done (who knows why). But that would be interesting in itself and highlight wasteful work in Lotus, or deficiencies in Forest.

One thing to fix to make this work that I can think of is the tipset cache, if we don't have all the tipsets available then we shouldn't be priming the cache on startup because it'll error.