bulk state upgrades on protocol upgrades

[raulk]

We should have a strategy to define how to upgrade system actors on protocol upgrades.

---

Hint: this likely does not integrate/interact with #717.

[raulk]

There is nothing too special about post-FVM builtin-actors migrations, except that:

1. The after CodeCIDs are specified in the bundle's manifest.
2. The system actor state embeds the manifest.

The logic for standard actor migrations has been implemented in a reusable form in filecoin-project/lotus#8815