We can fix it later (or now), but moving forward this will not do because the builder should be reusable, meaning that successive calls to `build()` should return `ThreadPoolExecutor` instances that do not share anything.

The main obstacle to this right now is the queue instance.  PR-99 includes a fix for this.  I'm happy to pull it out and make it a separate PR, or we let this one be merged as is and I can address it in PR-99.