Avoid duplicate timer in sliding window process function

[yunfengzhou-hub]

SlidingWindowKeyedProcessFunction.java:

            // TODO: Register at most one timer at a timestamp regardless of the number of the keys
            //  processed by the operator.
            while (triggerTime <= rowExpireTime) {
                ctx.timerService().registerEventTimeTimer(triggerTime);
                triggerTime += stepSizeMs;
            }