Improve content_ids handling by defered loading

[mdellweg]

The array_field can be huge and most of the time we don't need the values in python. Some uses can even be handled by clever SQL without ever channelling all the data through the network.

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[pedro-psb]

I was having a look into this, I think we could avoid hydrating content_ids for other methods as well, like added, removed and contains.

[acheng-01]

We have some overlapping changes here... Would you prefer that you merge this in first and then I rebase on the changes or should I undo my changes related to the deferment and have you rebase instead?