The `join_node_request` and `join_node_response` RPCs are added:
- `join_node_request` is sent from the joining node to any node in the
cluster. It contains some initial parameters that will be verified by
the receiving node, or the topology coordinator - notably, it contains
a list of cluster features supported by the joining node.
- `join_node_response` is sent from the topology coordinator to the
joining node to tell it about the the outcome of the verification.
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