net/dnscache: add overly simplistic DNS cache package for selective use

I started to write a full DNS caching resolver and I realized it was
overkill and wouldn't work on Windows even in Go 1.14 yet, so I'm
doing this tiny one instead for now, just for all our netcheck STUN
derp lookups, and connections to DERP servers. (This will be caching a
exactly 8 DNS entries, all ours.)

Fixes #145 (can be better later, of course)

stunner/stunner.go

@@ -12,6 +12,7 @@ import (
 	"sync"
 	"time"
 
+	"tailscale.com/net/dnscache"
 	"tailscale.com/stun"
 )
 
@@ -38,9 +39,9 @@ type Stunner struct {
 
 	Servers []string // STUN servers to contact
 
-	// Resolver optionally specifies a resolver to use for DNS lookups.
-	// If nil, net.DefaultResolver is used.
-	Resolver *net.Resolver
+	// DNSCache optionally specifies a DNSCache to use.
+	// If nil, a DNS cache is not used.
+	DNSCache *dnscache.Resolver
 
 	// Logf optionally specifies a log function. If nil, logging is disabled.
 	Logf func(format string, args ...interface{})
@@ -118,9 +119,6 @@ func (s *Stunner) Receive(p []byte, fromAddr *net.UDPAddr) {
 }
 
 func (s *Stunner) resolver() *net.Resolver {
-	if s.Resolver != nil {
-		return s.Resolver
-	}
 	return net.DefaultResolver
 }
 
@@ -192,9 +190,18 @@ func (s *Stunner) sendSTUN(ctx context.Context, server string) error {
 	}
 	addr := &net.UDPAddr{Port: addrPort}
 
-	ipAddrs, err := s.resolver().LookupIPAddr(ctx, host)
-	if err != nil {
-		return fmt.Errorf("lookup ip addr: %v", err)
+	var ipAddrs []net.IPAddr
+	if s.DNSCache != nil {
+		ip, err := s.DNSCache.LookupIP(ctx, host)
+		if err != nil {
+			return fmt.Errorf("lookup ip addr: %v", err)
+		}
+		ipAddrs = []net.IPAddr{{IP: ip}}
+	} else {
+		ipAddrs, err = s.resolver().LookupIPAddr(ctx, host)
+		if err != nil {
+			return fmt.Errorf("lookup ip addr: %v", err)
+		}
 	}
 	for _, ipAddr := range ipAddrs {
 		ip4 := ipAddr.IP.To4()