Remove custom stream, add a callback system with php_user_filter

The whole system of custom stream has been removed.

We now use a \php_user_filter where we inject a Guzzle Emitter and foreach message of the stream we create a new OutputEvent on "response.output".

This will allow to have multiple callback and a proper api.

Also, the chunk stream has been replace by the chunk filter which is defined in php (less maintenance).

Api of the attach call has been change, the callback must now always be set: this is more consistent as there is no need to attach a container to nothing.

ping @ubermuda would love to have you input on this between one of your flights :)