Modify transfer operators to handle more data

[mwallace582]

This addresses an issue where large data imports can result in filling
all available disk space and cause the task to fail.

Previously all data would be written out to disk before any was uploaded
to GCS. Now each data chunk is written to GCS and immediately freed.

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[boring-cyborg]

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[potiuk]

It looks nice, but I guess some tests will need to be updated too (let's see)

[mwallace582]

Thanks for taking a look and initiating the tests! I thought I had run through all the tests locally, but evidently not, as the failures appear to be related to my work. I'll get those tests fixed shortly.

[boring-cyborg]

Awesome work, congrats on your first merged pull request!

[potiuk]

Cool!