Add a MessageHeader field for monotonic_timestamp_time and extract it

We track the time from send -> republish already by logging delivery
timestamps.  When that time is sent back to the logger node, we actually
measure the time from republish -> logger node too, which would be
useful for timeline reconstruction.  Add a field to log that to and
extract any timestamps we find there.

Next step is to log it and use it again.

Change-Id: I36286903adfa418e830be68382b157794a16ff7a
diff --git a/aos/events/logging/logfile_utils.h b/aos/events/logging/logfile_utils.h
index 088ce73..94525cf 100644
--- a/aos/events/logging/logfile_utils.h
+++ b/aos/events/logging/logfile_utils.h
@@ -358,6 +358,9 @@
   monotonic_clock::time_point monotonic_remote_time = monotonic_clock::min_time;
   realtime_clock::time_point realtime_remote_time = realtime_clock::min_time;
 
+  monotonic_clock::time_point monotonic_timestamp_time =
+      monotonic_clock::min_time;
+
   SizePrefixedFlatbufferVector<MessageHeader> data;
 };