auto scaler policy notifies listener on max pool size reached

It allows a listener to be registered with AutoScaler, to be called if the "unbounded pool size" would consistently exceed the max allowed pool size for some period of time. 

My thinking is a demo can implement a listener that sends an SMS message to the operator (there are lots of SMS libraries and services out there to choose from). That gives a nice "open-loop" story when combined with an operator reconfiguring a policy (e.g. adjusting its max pool size).

There are various additional tweaks we can to do this (e.g. currently it just notifies the listener once; could repeatedly notify listener at regular intervals or when size has increased substantially since the last notification). But that is for future pull requests.