Add methods to retrieve max_id/since_id for collections.

[qnighy]

This is not a polished PR (which emerged from my personal, one-shot use), but I'm submitting this in case someone wants this feature.

This method is necessary when e.g. one wants to iterate through all followings or all followers of a user. I used this to extract all my followings in the friends.nico instance (because the server will be shut down soon)