Add debug application-state endpoint

This new endpoint allows clients to easily purge steering’s application state. It is primarily designed for use in integration tests as outlined in aura#387 (closed).

The endpoint is available in debug mode so that developers can inspect the schema. However, any request to it is denied unless the OPERATING_MODE environment variable is set to tests. This is a precaution for administrators that use the debug mode to analyze application issues in production.

Note: In originally proposed to add a /reset/ endpoint. I changed my mind about that because I find

curl -X DELETE http://steering/api/v1/debug/application-state/

much more expressive than

curl -X POST http://steering/api/v1/reset/