watcher: keep port state when no nameserver of a name answered (closes #193)

The port check removed the saved port state of every address no
configured name resolves to. A name whose nameservers all timed out
or failed is saved with no records, so its addresses looked gone and
their port state was removed; when the nameservers answered again the
port state was recorded afresh, and a port that opened or closed in
the meantime was not notified.

The removal now keeps an entry when one of the names saved on it is
configured and none of its nameservers answered on its last check. A
name whose nameservers answer with no addresses still loses it, and so
does a name no longer configured, whose saved state is never checked
again.

Model: opus-5-5

README.md

@@ -109,7 +109,9 @@ rejected.
   - Port transitioned from open to closed (or vice versa).
   - New IP appeared (from DNS change) and its port state was recorded.
   - IP disappeared (from DNS change) — noted in the DNS change
-    notification; port state for that IP is removed.
+    notification; port state for that IP is removed. When none of a name's
+    nameservers answered, its addresses are not known, so the port state saved
+    for them is kept.
 
 ### TLS Certificate Monitoring