Small corrections to ctrl-c behavior, tracing

[litlep-nibbyt]

• change snapshot not-changed message to trace logging level since it dominates the debug logs.
• ctrl-c now exits the process immediately without waiting. The rationale being that the user should be able to exit the app without waiting for nock processing to finish in the current event. I left the cancellation token inside of NockApp because we still want a way to gracefully shutdown the app from within the work() function.