Make a very fragile version of the watches work again

It works only if you:
- start an HTTP server (using 'irmin init -d')
- watch using the REST API (connect your browser to http://127.0.0.1:8080/watch/<path>)
- use the REST interface to do modify the database
  (for intance 'curl -d '{ "params": "xxx" }' http://127.0.0.1:8080/update/<path>/XXX)

I know how to make it work between backends, I'll do it later.