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/logging/logging_impl.cc b/aos/common/logging/logging_impl.cc
index f520b35..969c0ee 100644
--- a/aos/common/logging/logging_impl.cc
+++ b/aos/common/logging/logging_impl.cc
@@ -38,7 +38,7 @@
   Context *context = Context::Get();
 
   context->implementation = implementation;
-  global_top_implementation = implementation;
+  global_top_implementation.store(implementation);
 }
 
 void NewContext() {