[MERGE] Make _nodes_by_key lazy

[John Arbash Meinel]

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Robert Collins wrote:
> Robert Collins has voted resubmit.
> Status is now: Resubmit
> Comment:
> Rather than copying in the duplicate code, please refactor the base to
> have a helper method that does the stuff we want to reuse.

Done in attached.
I also factored out the "spill" code from add_node into a helper. It makes
things a bit easier to follow. I also added documentation for
_spill_mem_keys_to_disk.  Can you make sure that my understanding is correct?

John
=:->

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