votequorum: Don't send multiple callbacks when nodes join

This patch aligns the votequorum callbacks so that they are
the same as the quorum ones. Previously it was quite common
for votequorum to send one callback for every node in the cluster
when a single new node joined (because it sent one for every
nodeinfo message it received).

This new system makes much more sense in itself and being
consistent with the internal quorum is also an advantage!

Signed-off-by: Christine Caulfield <ccaulfie@redhat.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>