[2.0] Request service removal

This one is probably going to be controversial but I'd really think that removing the `request` service is a big step in the right direction.

Giving access to the Request object as a service is wrong for many reasons I won't reiterate here and of course it can cause problems if someone store a reference of it in another service. Instead, the `request_stack` should be injected and the Request object retrieved from it.

If you look at the patch, it's surprisingly easy enough to get rid of it probably because Silex/Symfony are already well aware of that problem (the main changes come from updating the tests).