request_constructor is now a Source method.
update_etags now takes &mut self instead of mut self.
update_etags now uses source setters methods instead of raw fields.
changed the naming of into_feed to to_feed according to rust convention.
Tried to have a seperate futures loop but it's too confusign having
too write a Trait2, functon2, etc version of everything and keep it together.
Futures are functional sort of, so the synchronous versioun can be removed.
It still needs a ton of work though to be ready, or even get near the perf
of of the sync+rayon version.
