Implement a callback when flowlet number of instances change

Fix for https://issues.cask.co/browse/CDAP-516.
Bamboo build: https://builds.cask.co/browse/CDAP-DUT156.
This would require documentation changes.
Yes I had a question about it, Is that what we do too for the ``@ProcessInput`` methods in flows? I couldn't track the code down to that...