Enable scraper's atomic feature so that Html is Send.

@OC000008373193 @OC000021106231 This would be an alternative to !583 (closed) which still prevents some code patterns but does not limit scalability at the cost of some efficiency.

This means instances of Html can stay on the stack even across await points but no references into them, e.g. ElementRef which is the typical result of evaluating a selector can be kept around, i.e. information must be fully extracted before or after fetching another document.

So while this is not a full solution, it may avoid most of the inscrutable compiler errors, still be sufficient to resolve the UIP issues with a bit of care but without a major rewrite at the cost of some limited performance degradation but without loss of scalability.