Transport Client: fixed the node retry mechanism which could fail without trying all the connected nodes

The RetryListener was notified twice for each single failure, which caused some additional retries, but more importantly was making the client reach the maximum number of retries (number of connected nodes) too quickly, meanwhile ongoing retries which could succeed were not completed yet.

The TransportService used to throw ConnectTransportException due to throwConnectException set to true, and also notify the listener of any exception received from a separate thread through the request holder.

Simplified exception handling by just removing the throwConnectException option from the TransportService, used only in the transport client. The transport client now relies solely on the request holder to notify of failures and eventually retry.

Closes #6829
Transport client: don't add listed nodes to connected nodes list in sniff mode

This commit effectively reverts e1aa91d , as it is not needed anymore to add the original listed nodes. The cluster state local call made will in fact always return at least the local node (see #6811).

There were a couple of downsides caused by putting the original listed nodes among the connected nodes:
1) in the following retries, they weren't seen as listed nodes anymore, thus the light connect wasn't used
2) among the connected nodes some were "bad" duplicates as they are already there and don't contain all needed info for each node. This was causing serialization problems for instance given that the node version was missing on the `DiscoveryNode` object.

Closes #7067