Add time-tracking to state management

[oubiwann]

• ~Create a namespace that won't get reloaded~
• [x] Define atoms for holding time data:
  • ~service start times~ (moved to #10)
  • [x] system start times
  • ~service stop times~ (moved to #10)
  • [x] system stop times
• [x] Define convenience functions for extracting data
  • [x] Get last start/stop/restart time
  • ~Get average start/stop/restart time~

Depends upon: #10, #11