Werk #10739: send notifications chronologically

Due to a bug in the notification spooler new spoolfiles were processed before old spoolfiles. This may lead to suprising results. If e.g. a host goes DOWN and UP in a short time interval the UP notification may arrive before the DOWN notification.

Now the notification spooler processes the notifications in the order of occurance, i.e. the DOWN notification will be sent before the UP notification.

SUP-1708

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