Add NodeEventLoopFactory

This lets us create event loops on separate nodes which can't
communicate with each other.  Next step is to add a message proxy
between them, then teach the logger to replay onto multiple nodes.

Change-Id: I06b2836365aea13d696535c52a78ca0c862a7b1e
diff --git a/aos/events/logging/logger.h b/aos/events/logging/logger.h
index 337109b..54b55d8 100644
--- a/aos/events/logging/logger.h
+++ b/aos/events/logging/logger.h
@@ -152,6 +152,7 @@
   std::vector<std::unique_ptr<RawSender>> channels_;
 
   std::unique_ptr<EventLoop> event_loop_unique_ptr_;
+  NodeEventLoopFactory *node_event_loop_factory_ = nullptr;
   EventLoop *event_loop_ = nullptr;
   TimerHandler *timer_handler_;