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/errors.go

@@ -22,6 +22,12 @@ var (
 		"reply is an error or a referral that leads no closer",
 	)
 
+	// ErrTruncated is the reason given for a reply too large for UDP
+	// whose retry over TCP failed.
+	ErrTruncated = errors.New(
+		"reply truncated and its retry over TCP failed",
+	)
+
 	// ErrIntercepted is returned when every root server refused a
 	// query. Root servers refuse no query, so the refusals came from
 	// something on the network answering in their place.