add missing header end comments

- ensures a consistent usage in header files
- also add a blank line after the copyright header where missing
- also remove orphan new-lines at the end of some files

src/allocators.h

@@ -252,4 +252,4 @@ struct zero_after_free_allocator : public std::allocator<T>
 // This is exactly like std::string, but with a custom allocator.
 typedef std::basic_string<char, std::char_traits<char>, secure_allocator<char> > SecureString;
 
-#endif
+#endif // BITCOIN_ALLOCATORS_H