Don't set IHttpContextAccessor by default

[DamianEdwards]

Setting up IHttpContextAccessor on every request is expensive and thus should probably be opt-in. Frameworks that require it (like MVC) can add it to the container as needed. If it's in the container, then we should set it up per request, otherwise, don't.

[rynowak]

We can drop our reliance on this and similar things in MVC. See aspnet/Mvc#3523