Swap direction of requests for propagating NAT state from Nexus

Today, dpd pulls NAT state from Nexus here:

https://github.com/oxidecomputer/dendrite/blob/4c6f4b8443e2394669eb6200a9e8e881f67740fd/dpd/src/rpw/mod.rs#L102

That is a small loop which fetches the latest generation of NAT entries from Nexus periodically, and ensures they're all added to the ASIC tables. dpd then stores the latest valid NAT state generation number, which Nexus separately pulls and uses to clean up old NAT state in the database in its own background task.

We'd like to switch the sense of this propagation, pushing the state from Nexus to dpd rather than pulling it. The main reason for this is that it enables easier updates. dpd would no longer be a client of Nexus's internal API, so not "client-side versioned" in the terminology of RFD 567.

This issue covers removing this task for fetching and ensuring the NAT state. There will be a separate issue in Omicron for adding propagation from Nexus to dpd.

Start in dpd/src/rpw/mod.rs around line 102 and read the task that periodically fetches and ensures NAT state from Nexus. Remove the fetching and ensuring work covered by this issue, while checking how the latest valid NAT state generation is stored or referenced. Done means dpd no longer performs this pull-based NAT state propagation; propagation from Nexus is covered by a separate Omicron issue.