Work done: * Added support for scale-in of operators. * Extended RoutingStrategyI interface to support collapse of replicas (only implemented for concrete stateless routing strategy). * Created new control tuple SCALE_IN to notify upstream operators that a downstream operator instance has been stopped. * Extended CoreRE and CoreProcessingLogic to support processing of scale-in control messages. * Implemented scale-in/unalert method on Infrastructure class to allow integration with the monitoring framework for SEEP.