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), and logs it with
the reason unless shutdown cut it short. A nameserver that answered no
type has failed, as before.
The watcher saves such a type in failedTypes, keeping the previous
check's records, leaves it out of the comparison with other nameservers
on that check, and compares the kept records with the next answer. With
nothing to keep, it is also in unknownTypes and not compared until it
answers. Change messages leave out what was not compared. A nameserver
whose A, AAAA or CNAME query failed is no answer when following a CNAME
or resolving addresses.

Model: opus-5-5

internal/resolver/resolver.go

@@ -31,11 +31,15 @@ type Params struct {
 }
 
 // NameserverResponse holds one nameserver's response for a query.
+// FailedTypes lists the record types whose query got no usable reply,
+// and Records holds nothing for them: their records are not known. When
+// no record type got one, Status and Error say the nameserver failed.
 type NameserverResponse struct {
-	Nameserver string
-	Records    map[string][]string
-	Status     string
-	Error      string
+	Nameserver  string
+	Records     map[string][]string
+	FailedTypes []string
+	Status      string
+	Error       string
 }
 
 // Resolver performs iterative DNS resolution from root servers.