[FLINK-2577] [streaming] Fix Stalling Watermarks when Sources Close
Before, when one source closes early it will not emit watermarks anymore. Downstream operations don't know about this and expect watermarks to keep on coming. This leads to watermarks not being forwarded anymore. Now, when a source closes it will emit a final watermark with timestamp Long.MAX_VALUE. This will have the effect of allowing the watermarks from the other operations to propagate though because the watermark is defined as the minimum over all inputs. The Long.MAX_VALUE watermark has the added benefit of notifying operations that no more elements will arrive in the future. This closes #1060
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