Make the Pager evolve to allow refresh.

[Natim]

The Pager API is exactly what one's need to fetch a list of Records.

However on time to time you wish to update the list, and it would be nice to be able to use the ETag to refresh the list without reloading the whole collection.

My understanding is that the Pager object is not far from being able to handle that.

Reference: http://docs.kinto-storage.org/en/stable/tutorials/synchronisation.html#polling-for-remote-changes