Uncacheable responses do not invalidate existing cache

[tejaede]

Steps:

1. Acquire JWT
2. Poll get_hazards with JWT
3. Cache valid responses as they arrive
4. JWT expires
5. Skip caching as the responses have an uncacheable status code.

Expected: The cache is invalidated. If it's the result of a client request, the uncacheable response is returned to the application layer. If it's a push promise, the next client request goes to the accelerator. Actual: If a cache exists, the application receives it in perpetuity.

[hthetiot]

See #55