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/build/aos.gyp b/aos/build/aos.gyp
index 3aaaf10..96cfc1b 100644
--- a/aos/build/aos.gyp
+++ b/aos/build/aos.gyp
@@ -7,9 +7,9 @@
 {
   'targets': [
     # A target for things used by the logging implementation (except die) to
-    # depend on that allows linking successfully with logging calls  but has no
-    # way to get initialized and so is basically useless unless something else
-    # links in the rest of the logging stuff.
+    # depend on that allows linking successfully with logging calls. However,
+    # executables containing targets that depend on this still need a dependency
+    # on logging somewhere or else they won't link.
     {
       'target_name': 'logging_interface',
       'type': 'static_library',