MEDIUM: session: use the pointer to the origin instead of s->si[0].end

MEDIUM: session: use the pointer to the origin instead of s->si[0].end

When s->si[0].end was dereferenced as a connection or anything in
order to retrieve information about the originating session, we'll
now use sess->origin instead so that when we have to chain multiple
streams in HTTP/2, we'll keep accessing the same origin.