Replace TimestampMapper::sorted_until with started()

Rebooting nodes will make sorted_until a pretty meaningless concept
since time can go backwards at the outer level of sorting.  The
only user (outside tests) is just using it to detect if sorting
has started.  Present that simpler interface instead to make the
contract easier to adhere to going forwards.

Change-Id: If91402a05677fdb07cd66cdb8e0066e31955e2d3
Signed-off-by: Austin Schuh <austin.schuh@bluerivertech.com>
diff --git a/aos/events/logging/logfile_utils.h b/aos/events/logging/logfile_utils.h
index 2219a07..b2a7f5d 100644
--- a/aos/events/logging/logfile_utils.h
+++ b/aos/events/logging/logfile_utils.h
@@ -518,9 +518,9 @@
   // node.
   void AddPeer(TimestampMapper *timestamp_mapper);
 
-  // Time that we are sorted until internally.
-  monotonic_clock::time_point sorted_until() const {
-    return node_merger_.sorted_until();
+  // Returns true if anything has been queued up.
+  bool started() const {
+    return node_merger_.sorted_until() != monotonic_clock::min_time;
   }
 
   // Returns the next message for this node.