Avoid using `defineProperties()` in MongooseBuffer constructor

[vkarpov15]

Very slow. Instead, we should have a class that extends from buffer, and make those properties non-enumerable. Just watch out for this bug: feross/safe-buffer#23

[vkarpov15]

Looks like buffers are now about 3x faster with this change:

const Benchmark = require('benchmark');
const mongoose = require('mongoose');
const mongoose55 = require('mongoose55');

const suite = new Benchmark.Suite;
 
// add tests
suite.
  add('5.4', function() {
    new mongoose.Types.Buffer('foo');
  }).
  add('5.5', function() {
    new mongoose55.Types.Buffer('foo');
  }).
  on('cycle', function(event) {
    console.log(String(event.target));
  }).
  on('complete', function() {
    console.log('Fastest is ' + this.filter('fastest').map('name'));
  }).
  run();

$ node bench.js 
5.4 x 269,260 ops/sec ±0.81% (92 runs sampled)
5.5 x 758,365 ops/sec ±0.73% (95 runs sampled)
Fastest is 5.5
$