Changes
BREAKING CHANGE: Since version 4.0, the client no longer polls for network
events at shutdown (during object destructor). This behaviour didn't give
enough control to the user in case of server issue, and could cause the script
to hang while terminating.
Starting from 4.0, programs MUST call flush() before shutting down, otherwise
some messages and callbacks may be lost.
## Features
- Add transactional producer support (#359, @nick-zh)