The idiomatic way to create derived data structures from data in the
Redux state store is to do:

  derivedData = transform(select(store.getState()))

Where the `select` function extracts the relevant fields from the state
and the `transform` function then computes the derived data. Both
`select` and `transform` can be trivially memoized to avoid unnecessary
recalculations.

This simplifies `root-thread` by avoiding inheritance from EventEmitter
and in future will make it easier to avoid rebuilding the thread if none
of the relevant application state has changed.