zerocopy: clean up TODOs

[mattkur]

Placeholder issue to track cleaning up TODO items left by #735 . This issue will be closed when we:

• decide to take action (or not!) on that cleanup, or
• have specific issues for the cases where we decide.

See the PR comments in #735 as to what the TODOs mean.

[smalis-msft]

I think leaving all these todo comments in for now, for the sake of getting the PR landed, is the right decision. We can farm out the cleanups to area owners afterwards, or even just tackle them ourselves.