Switch to milliseconds from unixtime seconds for the `createdAt` field on Tunnelbroker database message item related to the
[[ https://linear.app/comm/issue/ENG-1027#comment-0ef7fada | ENG-1027 comment ]].
Related task: [[ https://linear.app/comm/issue/ENG-1045/switch-to-milliseconds-from-seconds-in-tunnelbroker-message-time-of | ENG-1045 ]]