Fix slow V8 string operations in node 0.10

[ghost]

I managed to reproduce the issue mentioned in #205 with the script below. Hopefully, you'll be able to see the latency spikes when you run the script (from ~10ms to ~80ms).

Please run on node 0.10.

slow-set.js:

'use strict';

var Memcached = require('./');

var currentSize = 515000;
var endSize = 525000;
var inc = 100;

function createStringOfSize(size) {
  var buffer = new Buffer(size);
  buffer.fill('x');
  return buffer.toString();
}

function loopUntilComplete() {
  if (currentSize < endSize) return setKey(loopUntilComplete);
  process.exit(0);
}

function setKey(cb) {
  console.time('memcached.set');
  var key = 'default:' + 'foobydoo'; // slow
  //var key = 'default:foobydoo'; // fast
  var value = createStringOfSize(currentSize);

  memcached.set(key, value, 300, function(err) {
    if (err) throw err;
    console.timeEnd('memcached.set');
    console.log("Set cache key foobydoo with object of length " + currentSize);
    currentSize += inc;
    cb(err);
  });
}

var memcached = new Memcached(['localhost:11211'], {
  retry: 1000,
  timeout: 100,
  reconnect: 300000,
  poolSize: 100
});

loopUntilComplete();

[3rd-Eden]

Thanks for providing a reproducible test case for this and fixing all the things affected!