Add HttpContextAccessor

- Add [I]HttpContextAccessor and stop adding IContextAccessor as part of HostingServices
- Unify RequestContainer to use EnsureRequestServices

Review: Is Hosting the right home for IHttpContextAccessor?  Could also live in HttpAbstractions or RequestContainer...

cc @davidfowl @lodejard @divega 
Is AsyncLocal not available for ASPNET50, only ASPNETCORE50?