Add ClusterOptions.NewClient hook

[vmihailenco]

Replaces #1316

Can be used like this

rdb := redis.NewClusterClient(&redis.ClusterOptions{
	Addrs: []string{":7000", ":7001", ":7002", ":7003", ":7004", ":7005"},

	NewClient: func(opt *redis.Options) *redis.Client {
		opt.Limiter = getLimiter(opt)
		return redis.NewClient(opt)
	},
})

[walktall]

@vmihailenco Thanks! LGTM!

[ThomasJannaud]

Thanks!

for anyone needed a limiter per options:

opts.NewClient = func(opt *redis.Options) *redis.Client {
  opt.Limiter = _myCircuitBreaker[addr]  // you need to implement your own circuit breaker, one per address, careful of r/w access on the map too
  return redis.NewClient(opt)
}
client := redis.NewClusterClient(opts)