hydra: avoid duplicate proxies in spawn

[hzhou]

Pull Request Description

When user spawn with a host list in info hints, hydra creates new user_node_list, which results in launching new proxies. We only need the separate user_node_list for figuring out where to create the new processes, i.e. rankmap; but we should merge to the main node list to reuse the proxies.

For example, the following code will create multiple proxies even though all processes are launched on the local host:

    MPI_Info_create(&info);
    MPI_Info_set(info, "host", hostname);
    MPI_Info_set(info, "map-by", "ppr:1:node");
    MPI_Info_set(info, "bind_to", "core");

    for (int i = 0; i < 5; ++i)
    {
        MPI_Comm_spawn("./TestSpawn", argv, 1, info, 0, MPI_COMM_SELF, &null_comm, error_codes);
    }

[skip warnings]

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