refactor: expose API to perform intermediate read-only queries on state

[raulk]

There are several occasions in which the ABCI app needs to perform a query on the intermediate state prior to committing it. One such example is #1234.

Currently we're working around the lack of such an API by using modify_exec_state, which downgrades safety.

Let's implement the corresponding API so we can retain safety, and let's migrate illegitimate use cases to it.

Prior discussion

@cryptoAtwill Unfortunately the FVM executor does not expose read-only semantics through its execute_message API.

So we'd have to add a method to "fork" the executor/machine using the intermediate state tree root (which you can get from machine.state_tree()) and a BufferedBlockstore over the original Blockstore (machine.blockstore()), which we discard after we finish the query.

Originally posted by @raulk in #1234 (review)