watcher: send the inconsistency alert once per disagreement (closes #158)

detectInconsistencies alerted for neighbouring pairs of nameservers whose
records differed, on every DNS check, for as long as they differed. It now
also takes the previous hostname state, compares every pair of nameservers,
and alerts for a pair only when both were in that state with equal records.
The state loaded at startup is the previous state for the first check, so a
disagreement saved before a restart is not reported again. The choice of
pairs is tested on record data, and the alert itself through the hostname
change detection with the notifier stand-in and no resolver. The README
describes the new behaviour.

Model: opus-5-5

README.md

@@ -74,8 +74,12 @@ rejected.
     This is distinct from "responded with no records."
   - **NS recovery**: A previously-unreachable nameserver starts
     responding again.
-  - **Inconsistency detected**: Two nameservers that previously agreed
-    now return different record sets for the same hostname.
+  - **Inconsistency detected**: Two nameservers that agreed on the
+    previous check now return different record sets for the same
+    hostname. Every pair of nameservers is compared. The alert is sent
+    once for each such pair, on the check where they start to disagree,
+    and not again while they keep disagreeing, including after a restart.
+    If they agree again and later disagree, it is sent again.
 
 ### TCP Port Monitoring