[FLINK-9857] Delay firing of processing-time timers by 1 ms

[aljoscha]

Description from Jira issue:

The firing of processing-time timers is off by one. This leads to problems in edge cases, as discovered here (mailing list) when elements arrive at the timestamp that is the end of the window.
The problem is here (github). For event-time, we fire timers when the watermark is >= the timestamp, this is correct because a watermark T says that we will not see elements with a timestamp smaller or equal to T. For processing time, a time of T does not say that we won't see an element with timestamp T, which makes processing-time timers fire one ms too early.
I think we can fix it by turning that <= into a <.

I'm afraid there are no tests for this behaviour and there can't be tests for the new behaviour since we're dealing with processing time here.

R: @StephanEwen

[aljoscha]

I quickly talked to @StephanEwen directly and he agreed the fix is good so I'm merging.