Add the messages skipped when time skips an event

The first question ends up being "what messages triggered this?", so
let's just print them out

Change-Id: I1deb37037376544be0ac34c6e7588715a3674e16
diff --git a/aos/events/logging/logger.h b/aos/events/logging/logger.h
index a117b5a..e71f31d 100644
--- a/aos/events/logging/logger.h
+++ b/aos/events/logging/logger.h
@@ -504,6 +504,9 @@
     // filter to change the time offset estimation function.
     TimestampedMessage PopOldest(bool *update_time);
 
+    // Returns the oldest message (if it exists) non destructively.
+    const TimestampedMessage &PeekOldest();
+
     // Returns the monotonic time of the oldest message.
     monotonic_clock::time_point OldestMessageTime() const;