Add deduplication to the merge step
Page change events, and soon page refresh events, only have two possible actions to perform. These actions all overwrite what happened before, so there is no need to apply any update other than the last one received.
Analysis of existing CirrusSearch jobs suggests that by applying a 5 minute window we can reduce the rate of update jobs triggered by page-change events by almost 9%. Metrics have been added so we can verify this premise.
Bug: T325672 Change-Id: I927a62e44febca378722653c82e7d4e5c21f36dc