Add timestamp_extractor to simplify timestamp extraction

This is a simpler program which has fewer moving parts.
Sometimes, --skip_order_validation with log_cat isn't aggressive enough
to make the log readable because it is that busted.  timestamp_extractor
is good enough to parse those logs.

Change-Id: I8e5250cd149ef2f1b86384d61064d49da41f42c2
diff --git a/aos/events/logging/timestamp_plot.gnuplot b/aos/events/logging/timestamp_plot.gnuplot
index 566448a..7ad62da 100755
--- a/aos/events/logging/timestamp_plot.gnuplot
+++ b/aos/events/logging/timestamp_plot.gnuplot
@@ -28,6 +28,6 @@
      samplefile21 using 1:(-$2) title 'sample 2-1', \
      noncausalfile12 using 1:3 title 'nc 1-2' with lines, \
      noncausalfile21 using 1:(-$3) title 'nc 2-1' with lines, \
-     offsetfile using ((column(node1_index) - node1_start_time + (column(node2_index) - node2_start_time)) / 2):(column(node2_index) - column(node1_index)) title 'filter 2-1' with lines
+     offsetfile using ((column(node1_index) - node1_start_time + (column(node2_index) - node2_start_time)) / 2):(column(node2_index) - column(node1_index)) title 'filter 2-1' with linespoints
 
 pause -1