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/linux_code/logging/linux_interface.cc b/aos/linux_code/logging/linux_interface.cc
index 9889c07..cbc114e 100644
--- a/aos/linux_code/logging/linux_interface.cc
+++ b/aos/linux_code/logging/linux_interface.cc
@@ -42,7 +42,7 @@
 }  // namespace
 
 Context *Context::Get() {
-  if (my_context == NULL) {
+  if (__builtin_expect(my_context == NULL, 0)) {
     my_context = new Context();
     my_context->name = GetMyName();
     if (my_context->name.size() + 1 > sizeof(LogMessage::name)) {