resolver: try servers in a random order on each resolution (closes #138)

Every resolution walked the root servers in a fixed order, so
a.root-servers.net got every first query and its timeouts were paid on
every lookup. Each list of servers the resolver walks, the root servers
and the nameservers of each zone below them, is now walked in a random
order from the standard library's rand.Shuffle, chosen anew each time.
Failover is unchanged: a server that does not reply, or refuses, is
passed over for the next; any other reply, even a SERVFAIL, is used.
The shuffle is passed in, so the tests check the order with a seeded
source; which server a live query reached is not observable, so no
test fails if the walk stops shuffling.

Model: opus-5-5

internal/resolver/export_test.go

@@ -26,3 +26,16 @@ func (r *Resolver) QueryEachNS(
 ) (map[string]*NameserverResponse, error) {
 	return r.queryEachNS(ctx, nameservers, hostname)
 }
+
+// RootServerList exports rootServerList for testing.
+func RootServerList() []string {
+	return rootServerList()
+}
+
+// Shuffled exports shuffled for testing.
+func Shuffled(
+	servers []string,
+	shuffle func(n int, swap func(i, j int)),
+) []string {
+	return shuffled(servers, shuffle)
+}