wgengine/magicsock: bunch of misc discovery path cleanups

* fix tailscale status for peers using discovery
* as part of that, pull out disco address selection into reusable
  and testable discoEndpoint.addrForSendLocked
* truncate ping/pong logged hex txids in half to eliminate noise
* move a bunch of random time constants into named constants
  with docs
* track a history of per-endpoint pong replies for future use &
  status display
* add "send" and " got" prefix to discovery message logging
  immediately before the frame type so it's easier to read than
  searching for the "<-" or "->" arrows earlier in the line; but keep
  those as the more reasily machine readable part for later.

Updates #483

disco/disco.go

@@ -157,9 +157,9 @@ func parsePong(ver uint8, p []byte) (m *Pong, err error) {
 func MessageSummary(m Message) string {
 	switch m := m.(type) {
 	case *Ping:
-		return fmt.Sprintf("ping tx=%x", m.TxID)
+		return fmt.Sprintf("ping tx=%x", m.TxID[:6])
 	case *Pong:
-		return fmt.Sprintf("pong tx=%x", m.TxID)
+		return fmt.Sprintf("pong tx=%x", m.TxID[:6])
 	case CallMeMaybe:
 		return "call-me-maybe"
 	default: