More robust/explicit blockstore handles

[alexytsu]

https://github.com/anorth/fvm-workbench/pull/14#discussion_r1246197762

• [ ] Clone on blockstores can be ambiguous for example when cloning a MemoryBlockstore, the snapshot of state is preserved but subsequent updates to either blockstore are not visible in the other.
• [ ] Even when blockstore handles share underlying memory (SharedMemoryBlockstore) the underlying executor uses a BufferedBlockstore meaning changes can drift out of sync over time

Currently, it is advised to have short-lived scope and usage of the blockstore returned by Wrangler (and get fresh references when needed) but this still lives a subtle footgun in the current implementation.

[anorth]

We might explore patterns like accessing the Store from Bench/Wrangler in a callback, if that can help remove footguns.