Make promises harder to break

[adamnovak]

This should help with #5456 by making sure the update_file_stream() function can never result in an empty file if the writer crashes.

Previously, with the FileJobStore, we would truncate the old value before writing anything form the new stream. If the writer failed in the middle, the old value would be lost.

Now, all the job store implementations detect failure of the stream-writing context manager scope, and only commit the update if it succeeded.

I'm also making promise fulfillment log more, to see if we can catch #5456 again.

Changelog Entry

To be copied to the draft changelog by merger:

• Updating files in the job store by stream is now atomic: either the stream writer succeeds and the new content is visible, or something goes wrong and the old content is visible

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