handle the HTTP config changes via the HTTP api.

naive implementation for now. When a change is done in the config we are first
trying to restart the HTTP listener with the new config. If it fails we are
relaunching it with the old configuration.

caveats:

- we had to stop the old listener first due to the limitations given by cowboy.
- we try our best to return a 202 accepted response by spawning the change in
  the configuration but there is no garantee it arrives. It would be better to
defer the config change right after the response is sent.

fix #20