Processing timers can be triggered more efficiently

After FLINK-23208, the processing timers are triggered more efficiently but it can still be improved. (The performance can be tested with benchmark)

Currently InternalTimerService.onProcessingTime(long) polls a timer from processingTimeTimersQueue and register a new timer after the polled timer is triggered, which means timers with different timestamps will be registered for multiple times. This can be improved with codes below:

long now = System.currentTimeMillis() - 1
while ((timer = processingTimeTimersQueue.peek()) != null && timer.getTimestamp() <= now) {
    processingTimeTimersQueue.poll();
    keyContext.setCurrentKey(timer.getKey());
    triggerTarget.onProcessingTime(timer);
}

But due to the bug described in FLINK-23689, this change has conflicts with current implementation of TestProcessingTimeService.setCurrentTime(long), which causes a lot of tests to fail(e.g. InternalTimerServiceImplTest). Therefore, before working on this improvement, FLINK-23689 should be fixed firstly.