Expose intermediate state

[Stebalien]

At the moment, the FVM discards intermediate state and only flushes the final state-tree. However, for debugging (tracing, etc.), intermediate state can be useful.

We should provide a way to flush intermediate state. Something like:

1. A flag that causes intermediate state to be flushed. If we do it this way, we'll have to be careful to only use this flag when we've replaced the blockstore with a temporary blockstore.
2. A second blockstore for intermediate state (along with, possibly, a flag to actually use it).

[rjan90]

This is no longer a pre-requiste for filecoin-project/lotus#11635. And it is not clear if there is a big request for exposing intermediate state from our users.

[rvagg]

#2101