cpg: send single confchg event per group on joinlist

using a similar approach to

43bead364514e8ae2ba00bcf07c460e31d0b1765
"Send one confchg event per CPG group to CPG client"

which did the same for leave events on a network partition.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>