@Override
    public GraphComputer mapReduce(final MapReduce mapReduce) {
        this.mapReducers.add(mapReduce);
        return this;
    }
An {@code ExecutorService} that schedules up background work. Since a {@link GraphComputer} is only used once
for a {@link VertexProgram} a single threaded executor is sufficient.
tinkergraph-gremlin/src/main/java/org/apache/tinkerpop/gremlin/tinkergraph/process/computer/TinkerGraphComputer.java