Send one confchg event per CPG group to CPG client

We found that sheepdog will receive more than one confchg msg when
network partition occur. For example, suppose the cluster has 4
nodes: N1, N2, N3, N4, and they form a single-ring initially. After a
while, network partition occur, the single-ring divide into two
sub-ring: ring(N1, N2, N3) and ring(N4). The sheepdog in the ring(N4)
will receive the following confchg messages in turn:
Memb: N2,N3,N4 Left:N1 Joined:null
memb: N3,N4 Left:N2 Joined:null
memb: N4 Left:N3 Joined:null

This patch will fixed this bug, and the client will only receive one
confchg event in this case:
memb: N4 Left:N1,N2,N3 Joined:null

Signed-off-by: Yunkai Zhang <qiushu.zyk@taobao.com>
Reviewed-by: Angus Salkeld <asalkeld@redhat.com>