[DDC-357] Effective toOne joins

# What is this?

I've kind of rewriten querying of toOne relations. It is more data and query-count effective.

# How?

Let's demonstrate it on `CmsUser` and `CmsAddress` from tests models. Let's solve behaviour for toOne relations that are not mentioned in the query.

```sql
SELECT u FROM CmsUser u
```

## lazy + mapped by side

Already implemented, result is that CmsAddress would be proxy.

## lazy + inverse side 

`CmsUser` has `CmsAddress` relation that is mapped and owned by `CmsAddress` entity.

What has to happen? The identifier of `CmsAddress` cannot be loaded from users table nad has to be added automatic join for the table. Because it's lazy it will be hydrated as proxy, becase that is exactly what I've asked for.

If it would have been eagerly loaded, It would create 1+N queries problem that I'm trying to avoid with this. I have the relation as lazy, if I knew I would have needed it and wanned to optimized, I'd join it, but I didn't.

Result is therefore `CmsUser` entity + `CmsAddress` proxy 

## eager - both inverse and mapped by sides

The appropriate query component is generated with autojoin and auto selecting of the entity.

If it is self-referencing, the auto join is not generated becase it would cause infinite recursion.

# Why?

I've given this a lot of thought and tested it on our not-so-small application. We have unfortunately lot of entitiy relations that are mapped on the inverse side than we need to select, which is effectively killing performace [DDC-357](http://www.doctrine-project.org/jira/browse/DDC-357) 

I would have to go and list all the entities as partials to save performace creating such monsters as this

```php
$builder = $repository->createQueryBuilder("o")
	->leftJoin("o.voucher", "vu")->addSelect("partial vu.{id}")
	->leftJoin("o.address", "a")->addSelect("a")
	->leftJoin("o.restaurant", "r")->addSelect("partial r.{id, name}")
	->leftJoin("o.payment", "p")->addSelect("partial p.{id}")
	->leftJoin("o.rating", "rat")->addSelect("partial rat.{id}")
	->leftJoin("r.settings", "rs")->addSelect("partial rs.{id}")
	->leftJoin("r.address", "ra")->addSelect("ra")
	->leftJoin("r.position", "rp")->addSelect("partial rp.{id}");
# plus about five more just to make save performace
```

We all know that hydrating a large result set is a bottleneck and if I say the relation is lazy and I'm not joining it I **really don't want it to be joined with all it's data**!

Now imagine I just want to select few orders and render some data on the page.. I have tens of queries like this just because I have to. This is wrong that the ORM is tripping my feet like this.

# What now?

I know I have to solve theese: 

- [ ] more refactoring?
- [ ] more tests
- [ ] what to do with issue tests that now have changed behaviour?

Any suggestions?