redis cluster集群，cluster nodes获取节点信息缺失

[vicvon]

当redis cluster中主节点和对应的从节点为disconnected状态，通过redis_cluster::cluster_nodes获取到的节点信息会丢失disconnected节点的信息。 例如

29fe8907fda427676f1092c3a12e8f96fc9edfe5 127.0.0.1:16384 master - 0 1568875198663 100 connected 5462-10922
d4935a663f6d478a8ad45591a0290ad71c8a17bc 127.0.0.1:16380 slave,fail ec41b3d85747a55c0931b08613f01e28d73e9fe5 1568861725574 1568861724572 109 disconnected
cd3b773beb86818243bb20bc571c31dd3b366e13 127.0.0.1:16382 master - 0 1568875199666 99 connected 10923-16383
ec41b3d85747a55c0931b08613f01e28d73e9fe5 127.0.0.1:16383 master,fail - 1568862332282 1568862328668 109 disconnected 0-5461
cfcd4930b5016cd2847dc393dc2dd945d9ee308f 127.0.0.1:16381 myself,slave 29fe8907fda427676f1092c3a12e8f96fc9edfe5 0 0 96 connected
5870b5f0fa882019250aa1dcda4e80e000eec415 127.0.0.1:16385 slave cd3b773beb86818243bb20bc571c31dd3b366e13 0 1568875196652 99 connected

其中，127.0.0.1:16380和127.0.0.1:16383是一对节点，通过redis_cluster::cluster_nodes方法是获取不到这两个节点的信息的