Eliminate nullness errors from MongoDbIO

[kennknowles]

I didn't try to change the design. There are probably ways to redesign the internals a bit to consolidate the checks and get better messages. But as it is they would just be NPEs in random places, so this allows us to turn on the type checking that will protect any refactor from problems.

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[johnjcasey]

LGTM. I don't know MongoDbIO to know if any of these annotations might be inaccurate, but all the annotations follow from what existed before.

[kennknowles]

Yea I don't really know the IO other than the digging I did because I happened to be touching it. I believe I mostly just added annotations that were clearly necessary and checks that were also clearly necessary. And beyond that I trust the tests.

[kennknowles]

My main goal is to remove the blanket suppressions so that people don't apply them to new code produced by copy/paste.

[kennknowles]

I did also potentially add some "setting to null" that will allow things to be GC'd, but not in a way that will matter.