Change reducer layout to better match logic of counters, timers, events etc.

[marcus-grant]

The reducer layout should be this:

state = {
  counters: {
     // old clock state minus globalTimer
  },
  globalTimer: {
    period: (number),
    isActive: (bool),
  },
  eventLog: {
    byId: {
      id: (string),
      type: (string),
      finishedOn: (number, epoch time),
    },
    byType: {
      typeA: (array of Ids in order of most recently finished),
      // ... and so on ...
    },
  },
}

And there should be a parent reducer that handles all the combined logic of the separate sliced reducers.