TODO items.
o use real entropy to make random (ID, port) numbers more random.
o in production mode, do not free memory on exit. In debug mode, test leaks.
o profile memory allocation, and if performance issues, use special memory
  allocator. For example, with caches per thread.
o #define BIT_... different on bigendian and smallendian systems so that
  the htons on flags is not needed to send a message from the cache.