moved the logic from HttpKernel in FrameworkBundle to the HttpKernel component
[TwigBridge] added a render_* function to ease usage of custom rendering strategies

Here is the code you need to write when using the regular render
function for an ESI strategy:

    {{ render(path('path'), { strategy: 'esi' })
}}

And the same with the new render_* function:

    {{ render_esi(path('path')) }}
[HttpKernel] added a URL signer mechanism for hincludes
[HttpKernel] added missing phpdoc and tweaked existing ones
[HttpKernel] simplified and enhanced code managing the hinclude strategy
[FrameworkBundle] made some services private
[HttpKernel] made the strategy a regular parameter in HttpContentRenderer::render()
[FrameworkBundle] deprecated HttpKernel::forward() (it is only used once now and not part of any interface anyway)
made the content renderer work even when ESI is disabled or when no templating engine is available (the latter being mostly useful when testing)
moved the deprecation logic calls outside the new HttpContentRenderer class
moved the container aware HTTP kernel to the HttpKernel component