[RFC] Add Support for httpKernel POST

We currently don't subscribe the bridge to the data event of the react
request, so we never see the request body. This makes it impossible to
handle POST requests.

Changes so far:
* onRequest method: just store the react request and response objects
* move all the symfony logic to a new onData method
* bind onData to the request data event, which gets emitted by in the
  react http server just after the server request event.

There is some potential for inspecting the request and application in the
onRequest method and setting a nil state on the bridge so that the onData
event can bail out early for a bogus request, but I don't see any great
reason for doing that in addition to using Stack. We may as well just run
the application in onData.