`ProcessingTimeSessionWindows` and `EventTimeSessionWindows` should be generic

`ProcessingTimeSessionWindows` and `EventTimeSessionWindows` should be generic just like `DynamicEventTimeSessionWindows` and `DynamicProcessingTimeSessionWindows`.

now:
 

public class ProcessingTimeSessionWindows extends MergingWindowAssigner<Object, TimeWindow> {}

proposal:
 

public class ProcessingTimeSessionWindows<T> extends MergingWindowAssigner<T, TimeWindow> {}

If this ticket is ok to go, I would like to take it.