Support SRV servers in failover
    
    Adds a new failover API call fo_add_srv_server that allows the caller
    to specify a server that is later resolved into a list of specific
    servers using SRV requests.
    
    Also adds a new failover option that specifies how often should the
    servers resolved from SRV query considered valid until we need a
    refresh.
    
    The "real" servers to connect to are returned to the user as usual,
    using the fo_resolve_service_{send,recv} calls.
    
    Make SRV resolution work with c-ares 1.6