Long-running arti service reachability issues

@stefani and @jnewsome report that arti onion services become unreachable after some time. On the client side, the logs suggest the descriptor is not available at the expected HsDirs:

Jan 04 14:26:57.777 [info] handle_response_fetch_hsdesc_v3(): Received v3 hsdesc (body size 0, status 404 ("Not found"))

The obvious suspect is the publisher, but the problem could also be in the IPT manager (e.g. maybe the service is failing to establish its desired number of intro points), or in the way we're computing the HsDir rings (maybe we are publishing the descriptor, but to the wrong HsDirs).