[WIP] Kernel refactor

Currently, the handling of sub-requests (including ESI and hinclude) is mostly done in FrameworkBundle. It makes these important features harder to implement for people using only HttpKernel (like Drupal and Silex for instance).

This PR moves the code to HttpKernel instead. The code has also been refactored to allow easier integration of other rendering strategies (refs #6108).

The internal route has been re-introduced but it can only be used for trusted IPs (so for the internal rendering which is managed by Symfony itself, or by a trusted reverse proxy like Varnish for ESI handling). For the hinclude strategy, when using a controller, the URL is automatically signed (see #6463).

The usage of a listener instead of a controller to handle internal sub-requests speeds up things quite a lot as it saves one sub-request handling. In Symfony 2.0 and 2.1, the handling of a sub-request actually creates two sub-requests.

Rendering a sub-request from a controller can be done with the following code:

```jinja
{# default strategy #}
{{ render(path("partial")) }}
{{ render(controller("SomeBundle:Controller:partial")) }}

{# ESI strategy #}
{{ render(path("partial"), { strategy: 'esi' }) }}
{{ render(controller("SomeBundle:Controller:partial"), { strategy: 'esi' }) }}

{# hinclude strategy #}
{{ render(path("default1"), { strategy: 'hinclude' }) }}
```

The second commit allows to simplify the calls a little bit thanks to some nice syntactic sugar:

```jinja
{# default strategy #}
{{ render(path("partial")) }}
{{ render(controller("SomeBundle:Controller:partial")) }}

{# ESI strategy #}
{{ render_esi(path("partial")) }}
{{ render_esi(controller("SomeBundle:Controller:partial")) }}

{# hinclude strategy #}
{{ render_hinclude(path("default1")) }}
```