Allow Fixtures::persist() orverride

Sorry for opening a new PR. I did some mistake with my previous fork...

Here is my issue:
> In some case, I do not want to persist some objetcs.

> The only way I found to avoid object persistance is to override persist() method, but it's impossible because this method is private.

> Make it protected would solve this issue.

@tyx suggested :
> You can avoid to persist entity like this My\Fully\Qualified (local)

> See https://github.com/nelmio/alice#value-objects

Well, yes `(local)` option avoid entity persistance. But, with this option, the `load` method won't return those objects. And I need those objects to perform persistance on all or some of them.

I tryed to find a solution using a processor. But it seems impossible to avoid persistance by this way.