[BUG] Assumption that cache eviction does not happen

From #465: "The existing code assumes that all of the entities are in the cache once the Resolver runs. That process puts all of the entities and their grants into the cache so that by the time the Authorizer retrieves the grants, it fetches them from the cache."

It sounds like that the code really relies on the entities being available from the cache, which means that no eviction must happen. If eviction happens (which must be assumed to happen at any point in time to any cache entry) requests can fail at any time w/ misleading errors and/or wrong information.

Start by tracing the Resolver-to-Authorizer path described in the issue, focusing on how entities and grants are retrieved from the cache. Determine the expected behavior after an entry is evicted; done means requests do not fail with misleading errors or return incorrect information when cache entries disappear.

Written by the indexing model from the issue text.