Make log file header comparison more efficient

Use FlatbufferVector instead of DetachedBuffer because the resulting
copy is significantly more efficient than the reflection based copy that
was being done before.

Change-Id: I9721e014bb6911ae9cde8586313f8b511ff06c8f
diff --git a/aos/flatbuffer_merge.h b/aos/flatbuffer_merge.h
index 418ea5e..5535763 100644
--- a/aos/flatbuffer_merge.h
+++ b/aos/flatbuffer_merge.h
@@ -89,6 +89,12 @@
                            reinterpret_cast<const flatbuffers::Table *>(t2));
 }
 
+template <class T>
+inline bool CompareFlatBuffer(const aos::Flatbuffer<T> &t1,
+                              const aos::Flatbuffer<T> &t2) {
+  return t1.span() == t2.span();
+}
+
 }  // namespace aos
 
 #endif  // AOS_FLATBUFFER_MERGE_H_