Last downloaded date is updated on upload

We renamed the last_accessed field to last_downloaded as part of the work on NEXUS-12360, but the existing semantics don't entirely match what one would expect given the rename. In particular, there are a few situations where the Last Downloaded timestamp is updated when content is uploaded, and this doesn't match the expectations for it not to be.

Overall there are a couple of obvious solutions:

1. We can rename the field (again) to something more neutral.
2. We can stop updating this field for upload operations.

Given the expectations in NEXUS-12360 and conversations since then, I'm assuming we'll want to stop updating the field for upload operations and the like.