modules/http: graphs, prometheus metrics, websocks

* http embeds modified lua-http server code that
  reuses single cqueue for all h2 client sockets,
  this is also because the API in upstream is unstable
* http embeds rickshaw for real-time graphs over
  websockets, it displays latency heatmap by default
  and can show several other metrics
* http shows a world map with pinned recently contacted
  authoritatives, where diameter represents number
  of queries sent and colour its average RTT, so
  you can see where the queries are going
* http now exports several endpoints and websockets:
  /stats for statistics in JSON, and /metrics for
  metrics in Prometheus text format