Read actor CIDs from chain to carry through network upgrades

[snadrus]

Curio should be able to survive certain errors in calls to the chain by seeing that it's a CID problem and requesting the current actor CIDs.
In Curio, this should happen without restart.
At start-up, it should get the CIDs from the chain too.

Lotus Miner should also get this feature, but making it require a restart is reasonable.