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/prime/input/joystick_input.cc b/aos/prime/input/joystick_input.cc
index 6a6606e..7e2f0cd 100644
--- a/aos/prime/input/joystick_input.cc
+++ b/aos/prime/input/joystick_input.cc
@@ -70,7 +70,8 @@
           }
         }
       }
-
+    }
+    {
       using driver_station::ControlBit;
       if (data.PosEdge(ControlBit::kFmsAttached)) {
         LOG(INFO, "PosEdge(kFmsAttached)\n");