[ticket/12575] Pass service_collection instead of ContainerInterface.

PHPBB3-12575
[ticket/12575] Workaround for offsetExists seems not required for the Iterator.

PHPBB3-12575
[ticket/12575] Do not duplicate logic in service_collection_iterator.

PHPBB3-12575
[ticket/12575] Just forward service_collection::offsetGet() to container.

PHPBB3-12575
Merge pull request #2593 from bantu/ticket/12575

[ticket/12575] Cleanup service_collection_iterator