Fix concurrency issues in controlclient, ipn, types/logger (#456)

Signed-Off-By: Dmytro Shynkevych <dmytro@tailscale.com>

types/logger/logger.go

@@ -127,18 +127,23 @@ func RateLimitedFn(logf Logf, f time.Duration, burst int, maxCache int) Logf {
 // since the last time this identical line was logged.
 func LogOnChange(logf Logf, maxInterval time.Duration, timeNow func() time.Time) Logf {
 	var (
+		mu          sync.Mutex
 		sLastLogged string
 		tLastLogged = timeNow()
 	)
 
 	return func(format string, args ...interface{}) {
 		s := fmt.Sprintf(format, args...)
+
+		mu.Lock()
 		if s == sLastLogged && timeNow().Sub(tLastLogged) < maxInterval {
+			mu.Unlock()
 			return
 		}
-
 		sLastLogged = s
 		tLastLogged = timeNow()
+		mu.Unlock()
+
 		logf(s)
 	}