Leftover cyclic references are a source of memory leaks. While
our code is AFAIK unaffected by such leaks at the moment,
eliminating a potential source of bugs will make maintenance
easier.
We make the long_response API cycle-free by stashing the
callback into the NNTP object. However, callers will need
to be updated to get rid of the circular reference to $self.
We do that be replacing anonymous subs with name subroutine
references, such as xref_range_i replacing the formerly
anonymous sub inside hdr_xref.