optimize populate_startup_nodes method

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Hi antirez,

How do you do? I build the redis cluster for work recently. Then I read the source code in redis-rb-cluster seriously. Yeah, I am a Rubist. I find populate_startup_nodes method in cluster.rb that it can be optimized.

In populate_startup_nodes method, this line

@nodes.each{|n| @startup_nodes << n}

It run 16384 times @startup_nodes << n , I optimize this,

@cluster_slots.each do |item|
            ip, port = item[2]
            name = "#{ip}:#{port}"
            node = {
                :host => ip,
                :port => port,
                :name => name
            }
            @startup_nodes << node
        end

@cluster_slots is array, and its size is startup_nodes's size. My startup_nodes is

startup_nodes = [
        {:host => "127.0.0.1", :port => 7000},
        {:host => "127.0.0.1", :port => 7001},
        {:host => "127.0.0.1", :port => 7002}
    ]

@cluster_slots.each do |item| just run 3 times.

And I puts the @startup_nodes.uniq! , the result is same as old code.

Please review my code, if you think it is good, you can merge the pull request.

I'm happy to do this.