Use cursor pagination on ListProcessElements

This removes one expensive query on this endpoint, and it's easy to apply that pagination here as it's only used in 2 place:

• import task
• the frontend display

After some quick test, it seems even way better to increase the number of item per page here (200 seems doable)

This would require a benchmark to compare master vs. this branch performance (not integrated, a simple script to run the full pagination on a large query)