Assign IHttpContextAccessor in the default implementation of IHttpContextFactory

[davidfowl]

As part of #404, we should be setting the IHttpContextAccessor in CreateHttpContext

[benaadams]

Is CreateHttpContext per request or per connection? Per connection is fine; but per request setting an AsyncLocal incurs a new Dictionary cost per set.

[davidfowl]

It's per request. Nothing is changing there. We're just moving things around so that application code can remove it if required.

[benaadams]

Ah, kk