SAMZA-2801: fix the case when waterrmark is not aggregated when quorum is not met #1707
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Previous pr #1705 adds the support of preventing watermark stall during lagging upstream tasks. In testing we found a bug introduced by the logic that the aggregated watermark is not advancing if the smallest watermark have been updated, but the updated tasks within the idle timeout doesn't meet quorum. The fix is to keep the previous logic if the aggregated watermark has advanced, otherwise invoking the logic for computing quorum.
Also changed the quorum count to be a metric instead of logging, due to the potential large number of logs.