Simple work-queue broker/worker

1. worker polls GET /next
2. client submit work to POST / . The work is forwarded to a worker (from 1) with request id header
3. worker sends response to POST /response with the same request id, reply is forwarded to the client (2)

To send work:

curl -X POST http://localhost:9999/  -d "@request-body.txt"

This is reproduction code for:

• hyperium/hyper#1716
• hyperium/hyper#1717