public ForWorker whenWorkerFinishes(Phase phase, RunnableThatThrows action) {
      finishActions.computeIfAbsent(phase, p -> new ArrayList<>()).add(action);
      return this;
    }
Register {@code action} to run after this worker finished {@code phase}.
planetiler-core/src/main/java/com/onthegomap/planetiler/reader/osm/OsmPhaser.java