storage: track latest reported status and report on reconnect

[aljoscha]

Before, we would only pump what the dataflows reported back to the controller. Which would make it so that when a controller re-connects it doesn't get status messages for the latest state.

Now, we track the latest status update, per object, and also what has been reported, per object. And on re-connect we clear out what has been reported.

Motivation

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.