Multirack Join Completion response should return the bootstore generation number #11549

An operator completes a multirack join that triggers a request via nexus to sled-agent during rack adoption. Between the time the setup status was queried and that completion request is sent, the operator on the wicket side can upload a new RackNetworkConfig and bump the generation number of the bootstore. This race
is why we can't just return the generation number in the status request with the other information.

When the multirack join service receives the completion request it atomically stops allowing updates to the bootstore. We should therefore return the latest generation as the response to this request. However, since the the response can get lost we want to make this idempotent in case of retries. Therefore we'll need to memoize this value in the RssStatus::Completed field.

Note that if the sled-agent running the multirack join service crashes this state will be lost. That's already a problem though, and we aren't going to solve it here.