What's the proper state in which to leave a BlockchainPower after using it? The account needs to be unlocked, but there's no public interface for re-locking the account except to delete the power entirely.

Maybe we want to make it a context manager? I can imagine this:

with blockchain_power.unlock(password) as unlocked_power:
    unlocked_power.sign_message(whatever)
# end of context manager re-locks account