Replace map-reduce based QuerySet.sum/average with aggregation-based implementations

[wojcikstefan]

As discussed in #1021

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This change is

[wojcikstefan]

@lafrech @thedrow take a look at this when you get a chance - it's what we discussed at the bottom of #1021. Also @thedrow let me know how/if I should make changes to the documentation.

[wojcikstefan]

@thedrow thanks for the tuple suggestion - good point.

Is this a breaking change? I don't think it is but we should highlight this in the changelog anyway.

It passes all the tests that the old code passed and right now I can't think of a scenario it wouldn't cover.

Anything else we should cover before merging this?

Edit: ah, the changelog