TimestampableTrait: conflicting visibility

Private visibility of createdAt/ updatedAt properties will hide them from any class that does inherit the trait.

```php
class Base { use TimestampableTrait; } // is fine
class ComplexEntity extends Base {} // is not timestampable
```
The problem was hard to track for a project that required a magnitude of tables to share a small set of columns from traits and thus using a BaseEntity to set those up.

Might likely effect other traits/ super classes and ODM as well.