Feature: Allows overriding a getServer method for using a hashring

[sparkida]

Continuing the conversation from #158.

This is a very simple addition to the codebase that will make it easy to hook in a consistent hashing ring.

I'd also like to suggest using node-hashring for performance instead of the 3rd-eden HashRing. Most notably, 3rd-Eden's implementation uses a 128-bit MD5 and then reduces it to a 32-bit uint and node-hashring uses the 32-bit MurmurHash algorithm which offers tremendous performance gains.

Usage

const memjs = require('memjs');
const HashRing = require('node-hashring');
const servers = ['localhost:11211', 'localhost:11212'];
// build a map of server addresses to their index in the server list
const serverMap = {};
servers.forEach((server, index) => serverMap[server] = index);
const client = memjs.Client.create(servers.join(','));
// build the hashring
const hashRing = new HashRing(servers);
// override the getServer method
client.getServer = (key) => serverMap[hashRing.findNode(key)];

client.set('hello', 'world', {expires: 600}, function(err) {
  console.error(err);
});

client.get('hello', function(err, val) {
  if (err) {
    console.error(err);
  }
  console.log(val.toString());
});

[saschat]

This looks good to me, just one small comment.

[saschat]

Could you add the usage example to the comment?

[sparkida]

Just added the example, let me know if it needs changing, thanks!