Add remote timestamps and queue_index to context

This gives us the knobs to expose the remote timestamps and queue_index
to anything receiving the events.  The first use case is the logger.  It
can now log forwarded entries *without* having to make the
message_gateway responsible for logging this data.

Change-Id: Ie34dd040d270f4fa90ecd6e463069e1adca1818a
diff --git a/y2019/vision/server/server.cc b/y2019/vision/server/server.cc
index 99a3727..56aa289 100644
--- a/y2019/vision/server/server.cc
+++ b/y2019/vision/server/server.cc
@@ -165,7 +165,7 @@
                          &camera_frames) {
         while (drivetrain_status_fetcher.FetchNext()) {
           DrivetrainPosition drivetrain_position{
-              drivetrain_status_fetcher.context().monotonic_sent_time,
+              drivetrain_status_fetcher.context().monotonic_event_time,
               drivetrain_status_fetcher->x(), drivetrain_status_fetcher->y(),
               drivetrain_status_fetcher->theta()};