Weaken before non-blocking

[genio]

Each non-blocking call should weaken the reference to $self first.

# non-blocking request
$self->login(sub {
    my ( $sf, $err, $token ) = @_;
    return $sf->$cb($err,undef) if $err;
    my $url = Mojo::URL->new($sf->_instance_url)->path($sf->_path)->path("limits");
    $sf->ua->get($url, $sf->_headers(), sub {
        my ($ua, $tx) = @_;
        return $sf->$cb($sf->_error($tx->error, $tx->res->json),undef) unless $tx->success;
        return $sf->$cb(undef,$tx->res->json);
    });
});
return $self;

vs

# non-blocking request
Scalar::Util::weaken($self);
Mojo::IOLoop->delay(
    sub { $self->login(shift->begin); },
    sub {
      my ($delay, $err, $token) = @_;
      return $self->$cb($err, undef) if $err;
      return $self->ua->get(@req, $delay->begin);
    },
    sub {
      my ($delay, $tx) = @_;
      return $self->$cb($self->_error($tx), undef) unless $tx->success;
      return $self->$cb('', $tx->res->json);
    },
);
return $self;

[genio]

http://irclog.perlgeek.de/mojo/2015-09-28#i_11287191

if you don't store the global ioloop singleton in $self, there is no need to weaken for
Mojo::IOLoop->... methods
you weaken if there's a cycle somewhere, not preemptively