taskd.pl: hint that "ref-change" is obsolete
Girocco now delivers all of its ref change updates to taskd.pl in
a batch using the "ref-changes" message. It has done so now for
over a year.
Nothing in Girocco uses the "ref-change" message (single ref change
update at a time).
Include a warning message hinting that support for "ref-change" may
disappear in the future at some unspecified time for any non-Girocco
clients that may be sending ref change updates directly to Girocco's
taskd.pl socket.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>