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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

When would the connection timeout increase?

Every time the timeout hits, the timeout value of the next connection try will increase. In my implementation, the timeout will hit iff a connection stays at the `connecting` state longer than the timeout value, as indicated by ClusterConnectionStates.NodeConnectionState. The connection state of a node may change from `connecting` to 

1. `disconnected` when SocketChannel.finishConnect() throws IOException.
2. `connected` when SocketChannel.finishConnect() return TRUE.

In other words, the timeout will hit and increase iff SocketChannel.finishConnect() keep returning false before the timeout arrives, which means, for example, network congestion, failure of the ARP request, packet filtering, routing error, or a silent discard may happen.

Compatibility, Deprecation, and Migration Plan

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