When a new node joins the cluster we need to be sure that it's IP
is known to all other nodes. In this patch we do this by waiting
for the IP to appear in raft_address_map.
A new raft_topology_cmd::command::wait_for_ip command is added.
It's run on all nodes of the cluster before we put the topology
into transition state. This applies both to new and replacing nodes.
It's important to run wait_for_ip before moving to
topology::transition_state::join_group0 since in this state
node IPs are already used to populate pending nodes in erm.
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