Keep track of "dirty" actors in the state tree

[Stebalien]

Currently, when we read an actor, we stick it in the state-tree cache. But that means we try to write it back to the blockstore at the end of the block, even if it hasn't changed.

We should add a dirty bit to to the cache so we don't do this.

[Stebalien]

Specifically, we need to distinguish between dirty and just "cached" in:

https://github.com/filecoin-project/ref-fvm/blob/6f3db40ad7108e27146d0b9efdf7c64107507839/fvm/src/state_tree.rs#L306

and

https://github.com/filecoin-project/ref-fvm/blob/6f3db40ad7108e27146d0b9efdf7c64107507839/fvm/src/state_tree.rs#L326

Furthermore, we should be caching "nonexistence".

[raulk]

@mriise this was assigned to you. Do you plan on acting on it, or should we reassign?