Adding new Siddhi event model for Siddhi 3.0.0 release. As per the new model Siddhi accepts Event objects and internally at execution level they are converted to StreamEvent/StateEvent objects. Also this contain MetaStreamEvent and MetaStateEvent classes to hold information relevant for event conversion between Event and Stream/StateEvent. EventConverter is added to carry out event conversions. Also StreamEventPool is used to prevent new object creation.