Implement the introduction point failure cache for proposal 224. This can be found in the legacy code in **src/or/rendcache.c**. See `strmap_t *rend_cache_failure` and functions starting with `rend_cache_failure_*`. We could re-use it but refactoring would be needed since some of the functions expects a `rend_service_descriptor_t` and they are indexed by `service_id` (onion address).