raft_group0 used to register RPC verbs only on shard 0.
This worked on clusters with the same --smp setting on
all nodes, since RPCs in this case are (usually)
processed on the same shard as the calling code,
and raft_group0 methods only run on shard 0.
A new test test_nodes_with_different_smp was added
to identify the problem.
Fixes: #12252