[Notification Aggregator] Counting _doneWeights is over-complicated

[Piotr Jasiun]

Working on #12955 we agreed that, since counting _doneWeights and _totalWeights is pretty simple and can be stored in redundant variables, counting _doneWeights is over-complicated and can be a reason of the future bugs. Tasks should give its done wights to aggregator and it should could sum of them, when it is needed.

Also with the current implementation it is not possible to update with negative weight, for no apparent reason.

Since this changes may causes some changes in API this ticket should be done before releasing 4.5.0.