Flattened the Request structure.

Currently we have a `Request` object, composed in this way:
* `session`
* `channel`
* `msg`

This reduces the ability to compose applications using streams: each microservice __must__ know about the `Request` structure.

This pull-request explores an alternative implementation for `Request`, by adding two 'hidden', (e.g. `hasOwnProperty` return false) properties, `_session` and `_channel`.

This might simplify building applications. Please review.