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)

derp/derphttp/derphttp_client.go

@@ -26,6 +26,7 @@ import (
 	"time"
 
 	"tailscale.com/derp"
+	"tailscale.com/net/dnscache"
 	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
 )
@@ -37,7 +38,8 @@ import (
 // Send/Recv will completely re-establish the connection (unless Close
 // has been called).
 type Client struct {
-	TLSConfig *tls.Config // for sever connection, optional, nil means default
+	TLSConfig *tls.Config        // for sever connection, optional, nil means default
+	DNSCache  *dnscache.Resolver // optional; if nil, no caching
 
 	privateKey key.Private
 	logf       logger.Logf
@@ -137,11 +139,23 @@ func (c *Client) connect(ctx context.Context, caller string) (client *derp.Clien
 		}
 	}()
 
+	host := c.url.Hostname()
+	hostOrIP := host
+
 	var d net.Dialer
-	log.Printf("Dialing: %q", net.JoinHostPort(c.url.Hostname(), urlPort(c.url)))
-	tcpConn, err = d.DialContext(ctx, "tcp", net.JoinHostPort(c.url.Hostname(), urlPort(c.url)))
+	log.Printf("Dialing: %q", net.JoinHostPort(host, urlPort(c.url)))
+
+	if c.DNSCache != nil {
+		ip, err := c.DNSCache.LookupIP(ctx, host)
+		if err != nil {
+			return nil, err
+		}
+		hostOrIP = ip.String()
+	}
+
+	tcpConn, err = d.DialContext(ctx, "tcp", net.JoinHostPort(hostOrIP, urlPort(c.url)))
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("Dial of %q: %v", host, err)
 	}
 
 	// Now that we have a TCP connection, force close it.