prop224: Client introduction point failure cache
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
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).