[BUGFIX] Avoid circular reference of COR and ServerRequest

While the initial idea to store the current ContentObjectRender
(cObj) instance in a request attribute, to pass it along to code
that requires both, the request and a cObj, is solid, we missed
that cObj itself does not require to hold the request as attribute.

The patch changes this: The request attribute is only set
for ContentObjects, as these are actually responsible for
rendering and eventually evaluating a request or the cObj
instance.

This makes sure, that an instance of cObj is available for
Extbase plugins as well as for Fluid view helpers, which can
access the request via RenderingContext.

By changing the concept slightly, all places that previously added
the cObj to the request can be removed in favor of doing so only
in AbstractContentObject and ContentObjectRenderer::callUserFunction.
The latter is at least required for TypoScript using the old way of
calling Extbase plugins with USER and it's userFunc property.

The circular refere...