[Internal] verify node is no longer in ClusterState before disconnecting it (after a ping failure)

After a node fails to respond to a ping correctly (master or node fault detection), they are removed from the cluster state through an UpdateTask. When a node is removed, a background task is scheduled using the generic threadpool to actually disconnect the node. However, in the case of temporary node failures (for example) it may be that the node was re-added by the time the task get executed. We should check for that.