DDC-3120 - PHP 5.6-RC3 compatibility

This PR replaces the current serialization hack used by doctrine with the polyfill provided by [`doctrine/instantiator`](https://github.com/doctrine/instantiator).

This will allows users of PHP 5.6 to rely on entities extending internal PHP classes again.
it will be `null` after `unserialize()` due to how `__sleep` works here