Add support for consistent hashing

[EnchanterIO]

From a quick look it seems like deciding to what node the key goes is based on crc32 hash and modulo.

func (ss *ServerList) PickServer(key string) (net.Addr, error) {
	ss.mu.RLock()
	defer ss.mu.RUnlock()
	if len(ss.addrs) == 0 {
		return nil, ErrNoServers
	}
	if len(ss.addrs) == 1 {
		return ss.addrs[0], nil
	}
	bufp := keyBufPool.Get().(*[]byte)
	n := copy(*bufp, key)
	cs := crc32.ChecksumIEEE((*bufp)[:n])
	keyBufPool.Put(bufp)

	return ss.addrs[cs%uint32(len(ss.addrs))], nil
}

But apparently (TIL) there is a better practice by using consistent hashing.

Unfortunately, this particular approach suffers from a fatal flaw due to the way that 
modulo works. As the number of cache nodes scales up, most hash keys will get 
remapped to new nodes with empty caches, as a side effect of using modulo. You can 
calculate the number of keys that would be remapped to a new cache node by dividing 
the old node count by the new node count. For example, scaling from 1 to 2 nodes 
remaps half (½) of your cache keys; scaling from 3 to 4 nodes remaps three-quarters 
(¾) of your keys; and scaling from 9 to 10 nodes remaps 90 percent of your keys to 
empty caches

From https://d0.awsstatic.com/whitepapers/performance-at-scale-with-amazon-elasticache.pdf

Am I understanding correctly that gomemcache doesn't have consistent hashing?

Thanks

[lovelock]

Feel free to extend it like I do https://github.com/lovelock/gomemcache