Unique contacts, bugfix in fbpost and better handling of avatars and cached data in privacy_image_cache

The new API is using the unique contacts. This patch is filling them while importing items.

In fbpost there was a bug that comments weren't queued correctly.

In privacy_image_cache now avatars from wall-to-wall posts are cached as well. Additionally pictures are stored at one place. The current situation was, that cached data was stored in the database at all the time - even if it was stored in a file as well. That was space wasting.