Snapshot effect for mutable state machines

[hairyhum]

If a state machine is using external mutable state emitting {release_cursor, Index, State} does not make much sense. Especially because effects will be processed after a batch of commands are applied, which means that the external state will change at that point.
A new type of effect, for example ok_to_snapshot may be introduces to capture the current state with the current index.
This will not provide a possibility to capture the specific index and state, but will be faster and may be enough for many mutable state machines.