Closing behavior is buggy

```
  2) Closing behavior  should close cleanly 2:
     Uncaught Error: pool is draining and cannot accept work
      at Pool.acquire (node_modules/generic-pool/lib/generic-pool.js:386:11)
      at lib/render.js:189:26
```

https://travis-ci.org/mapbox/mapnik-swoop/jobs/189853314#L1508