User session grouping

You are given `events`, a list of [user, timestamp] pairs (timestamps are integer seconds, NOT necessarily globally sorted, but per user they are non-decreasing in the order given). Group each user's events into sessions: a new session starts whenever the gap from the previous event of that same user exceeds `idle` seconds. Return a list of [user, session_count] sorted by user ascending. A user's first event always starts a session.