sled-agent: Fix races when starting switch zone in a4x2#9297

jgallagher merged 2 commits intomainoxidecomputer/omicron:mainfrom

john/sled-agent-underlay-info-while-starting-switch-zoneoxidecomputer/omicron:john/sled-agent-underlay-info-while-starting-switch-zoneCopy head branch name to clipboard