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1. Add a new HashSet property ConnectingNodes keeping all the connecting node ids.
2. Will expose a public API that returns the ConnectingNodes mentioned in #1, helping the NetworkClient process the timeout iteration.
3. State transition:
   1. ClusterConnectionStates.connecting() will add the node id to ConnectingNodes
   2. ClusterConnectionStates.ready() will remove the node id to ConnectingNodes
   3. ClusterConnectionStates.disconnected() will remove the node id from ConnectingNodes

Relation to

Compatibility, Deprecation, and Migration Plan

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