Make cutoff times stable and edittable in the UI

[AlanSl]

• [x] Identify cause of inconsistent application of cutoff times
• [x] Refactor useDatastore to keep all persistent variables in react hooks
• [x] Add tests to useDatastore
• [x] Update mock data server to send a new runtime in response to config signals changing cutoff time and state duration settings
• [x] "Fade out" connection table items that approach cutoff time
• [x] Add controls to the UI that send signals changing cutoff time and state duration
• [x] Tweak presentation of fading of drifting-out closed connections