The patch adds new marker for a verb - [[ip]] that means that for this
verb ip version of the verbs needs to be generated. Most of the verbs
do not need it.
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.