resolver, watcher: a record type whose query fails keeps its previous records (closes #231)

The resolver lists in FailedTypes each record type whose query to a
nameserver got no usable reply: none after two tries, an error reply, a
referral, or a truncated reply whose TCP retry failed. Records holds
nothing for such a type, never none or the part that fit. A nameserver
that answered no type has failed, as before. The watcher keeps the
previous check's records for a failed type; when that check did not know
them either (first check, new or failing nameserver), the type is saved
in failedTypes and left out of record and inconsistency comparisons.
ResolveIPAddresses no longer counts a nameserver whose A, AAAA or CNAME
query failed as an answer.

Model: opus-5-5

internal/state/state.go

@@ -44,9 +44,14 @@ type DomainState struct {
 	LastChecked         time.Time           `json:"lastChecked"`
 }
 
-// NameserverRecordState holds one NS's response for a hostname.
+// NameserverRecordState holds one NS's response for a hostname. A record
+// type whose query failed keeps the records saved by the previous check.
+// FailedTypes lists such types whose records the previous check did not
+// know either, as when the nameserver was new or failing then: Records
+// holds nothing for them.
 type NameserverRecordState struct {
 	Records     map[string][]string `json:"records"`
+	FailedTypes []string            `json:"failedTypes,omitempty"`
 	Status      string              `json:"status"`
 	Error       string              `json:"error,omitempty"`
 	LastChecked time.Time           `json:"lastChecked"`