NIFI-5544: GetMongo refactored

[zenfenan]

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[MikeThomsen]

Overall looks solid, but we can't be loading the data into a list like that.

[MikeThomsen]

I think I actually refactored this is another one of my Mongo commits... :-D

[MikeThomsen]

We can't load the result set into a list like this because it's easy for a Mongo result set to overflow the heap.

[zenfenan]

Fair point. I'll work on that.

[MikeThomsen]

+1 LGTM and integration tests ran fine.