[WIP] Lazy criteria for ManyToMany collection

This continues my previous work on making Criteria most efficient.

Currently we are wrapping matching calls on repositories and matching calls on EXTRA_LAZY associations around a LazyCriteria. However, ManyToMany are still completely loaded: https://github.com/doctrine/doctrine2/blob/master/lib/Doctrine/ORM/PersistentCollection.php#L874

This is still problematic from a performance point of view because count, contains... cannot be optimized. I think the solution is similar to previous one, hence creating a Lazy collection for that kind of associations.

However, this is really tricky to do because of the whole mess inside the persisters (can't wait for them to be completely refactored, it's getting really hard to maintain this mess :p).