def setnx(msg_id, suffix, value)
      with_failover { redis.setnx(key(msg_id, suffix), value) }
    end
def setnx(msg_id, suffix, value)
      with_failover { redis.setnx(key(msg_id, suffix), value) }
    end
store a (key,value) pair with given <tt>suffix</tt> for given <tt>msg_id</tt> if it doesn't exists yet.