BUG-2049: DataStore failure in StatisticsManager

!WORKAROUND ONLY

- during node reconciliation statsManager could join together 2 similar
  sets of statistics notifications and hit duplicate values during
  processing - here we just skip those duplicates in order not to loose
  stats working thread and finish actual multipart

Change-Id: I526ab5439055ac565bfaa08e84e5043e8764846a
Signed-off-by: Michal Rehak <mirehak@cisco.com>