Add ability to reset an application

**Is your feature request related to a problem? Please describe.**
Instead of creating a new application every time, what if we could reset some "state" and "application-y" things about it?

**Describe the solution you'd like**
Things that could be helpful:

- `.set_state(...)`
- `.set_identifiers(...)`
- `.set_next_action(action_name)`

**Describe alternatives you've considered**
You just reinstantiate the graph each time.

**Additional context**
This is mainly an edge case around stateless web-services. Do you create new, or reset the state?