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)

wgengine/magicsock/magicsock.go

@@ -31,6 +31,7 @@ import (
 	"tailscale.com/derp"
 	"tailscale.com/derp/derphttp"
 	"tailscale.com/interfaces"
+	"tailscale.com/net/dnscache"
 	"tailscale.com/netcheck"
 	"tailscale.com/stun"
 	"tailscale.com/stunner"
@@ -638,6 +639,7 @@ func (c *Conn) derpWriteChanOfAddr(addr *net.UDPAddr) chan<- derpWriteRequest {
 			c.logf("derphttp.NewClient: port %d, host %q invalid? err: %v", addr.Port, host, err)
 			return nil
 		}
+		dc.DNSCache = dnscache.Get()
 		dc.TLSConfig = c.derpTLSConfig
 
 		ctx, cancel := context.WithCancel(context.Background())