made sure everything's thread safe

Pretty much everything already was, but I tweaked a few things to make
sure they stay that way and added various comments about that fact. Also
cleaned up a few things along the way and removed SafeMessageBuilder
because it wasn't.
diff --git a/aos/common/time.h b/aos/common/time.h
index 846366c..0440d91 100644
--- a/aos/common/time.h
+++ b/aos/common/time.h
@@ -205,8 +205,7 @@
   }
 
   // Enables returning the mock time value for Now instead of checking the
-  // system clock.  This should only be used when testing things depending on
-  // time, or many things may/will break.
+  // system clock.
   static void EnableMockTime(const Time &now = Now());
   // Calls SetMockTime with the current actual time.
   static void UpdateMockTime();