Notify.pm: make ref_change function private

Notify.pm: make ref_change function private

The "ref-change" message to taskd.pl has been obsolete for some
time.  However, the implementation was still directly calling the
Girocco::Notify::ref_change function.

We want all notifications to come through the one canonical
Girocco::Notify::ref_changes function.

Make Girocco::Notify::ref_change truly private (it's still used as
part of the implementation of the ref_changes function) and modify
taskd.pl to submit obsolete one-ref-at-a-time notifications received
using the obsolete "ref-change" message through the
Girocco::Notify::ref_changes function instead.

With a single point of entry in Notify.pm, future extensions are
greatly simplified.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>