Currently, it's impossible to connect using unix socket

Hello and thanks for this library 😎

Right now message object (msg) is being reused for both send and receive in perform functionality.

There are some cases (e.g. Version()) in which the message is being sent to all nodes.

Because of that after the first send-receive the message msg is "mutated" as answer and thus not valid message to send in the next request.

Steps to reproduce

memcached -p 11289
memcached -p 11290
memcached -p 11291

And run

c := NewMC("localhost:11289 localhost:11290 localhost:11291", "", "")
vers, err := c.Version()

(I will open a PR for that with a fix suggestion)

When n servers have isAlive set to false the following line attempts to read out of the bounds of the c.servers array:

c.servers[idx+i%nServers]

If idx==1, i==1 and nServers==2 then 1+1%2 == 2

Should be rewritten as

c.servers[(idx+i)%nServers]

I'll raise a PR shortly.

Memcache has no commands to view the index, but a small amount of index content can be viewed through CacheDump. Can this function be added?

Please see：
https://github.com/Songmu/go-memcached-tool/blob/3b74077e768b5fbe832d82cc3544c1ff0b5736c7/memdtool.go

I have tried to find documentation that talks about the concepts as shown in the picture. But I still don't quite understand the 2 concepts. Can someone explain to me more about them?

Active Connections: 
New Connection Rate Last Hour
https://gyazo.com/c444d88dcd67ffa155470ab913f541d8

every time Get() a key, server.perform create a Ticker ,so my CPU rate is grow slowly. As my option, should be used time.After().

type Config struct {
	Hasher     hasher
        ...

type hasher interface {
	update(servers []*server)
	getServerIndex(key string) (uint, error)
}

now Hasher should implement an interface with private methods with private types like *server so the only way to define a Hasher is to use a NewModuloHasher() function.

the problem is that Hasher field is public, but i cant really use it.

i purpose to change the interface this way:

type Hasher interface {
	Update(servers []strings)
	GetServerIndex(key string) (uint, error)
}

this will let us to make custom implemenation of Hasher interface

When sending requests, the current behaviour is always performing authentication when connecting to server. This does not make sense when either the username or the password is empty.

A proposed fix can be adding below checking before https://github.com/memcachier/mc/blob/master/server_conn.go#L91

    if sc.username == "" || sc.password == "" {
        return nil
    }