only cancel recent duplicate pending messages

a created_at scope will limit the search at most two
partitions

closes #CORE-2381

Change-Id: I4893eaad14198385e88dc7455200198eac550e24
Reviewed-on: https://gerrit.instructure.com/178647
Reviewed-by: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>
Tested-by: Rob Orton <rob@instructure.com>

lib/notification_message_creator.rb

@@ -297,6 +297,7 @@ class NotificationMessageCreator
       by_name(@notification.name).
       for_user(@to_users + @to_channels).
       cancellable.
+      where("created_at BETWEEN ? AND ?", Setting.get("pending_duplicate_message_window_hours", "6").to_i.hours.ago, Time.now.utc).
       update_all(:workflow_state => 'cancelled')
   end