[DependencyInjection] inlined factory not referenced

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

While working with the DI I encountered a `You have requested a non-existent service "xxxxx"` exception. 
Research it seems that a private reference was inlined but the private factory that was also referencing to the service was ignored.

A example of the problem:
```XML
        <service id="manager"
                 class="\stdClass"
                 factory-method="getX"
                 factory-service="factory"
                 public="false">
            <argument>X</argument>
        </service>
        <service id="repository"
                 class="\stdClass"
                 factory-method="getRepository"
                 factory-service="manager"
                 public="false">
            <argument>X</argument>
        </service>
        <service id="storage" class="\stdClass" public="false">
            <argument type="service" id="manager"/>
            <argument type="service" id="repository"/>
        </service>
```

What happens before the patch:
1. repository get's inlined
2. manager get's inlined for the first argument
3. manager get's removed since there was no reference.

After the first commit the following will happen:
1. repository get's inlined 
2. manager get's inlined for the first argument 
3. manager will not be removed since the inlined repository still references manager

This introduced a smell since InlineServiceDefinitionsPass was still inlining the manager for the first argument. 

To fix this I have chosen that not inline factories if they are used more then once by the same definition.

So after the second commit the following will happen:
1. repository get's inlined 

Personally I feel that the InlineServiceDefinitionsPass patch isn't the best possible one but that a different fix would probably mean breaking BC so it's probably a good idea to look at this for Symfony 3.0.