Allow cache invalidation on policy update (or custom event)

[anderseknert]

Since caching is essential for good performance it makes sense to cache aggressively - both in terms of cache size but also in keeping the TTL of cached objects high. The downside of this is of course that policy (or policy data) changes take a long time to have an effect.

To tackle this while maintaining high performance, we should look into the possibilities to invalidate the cache on changes.

Some options to explore could be:

• allow programmatic invalidation at some predefined endpoint.
• hook into the currently unimplemented [add|get|remove]Acls methods on the authorizer interface to do cache invalidation.
• use OPA's status API mechanism to know when policy or data changed.