Fixes #3355.

…ationData` as well

…ound

Only code motion. No changes in functionality.

No changes in functionality, just renamings.

…agination that failed under our feet

When a timeline reset happens while we're back-paginating, the event
cache method to run back pagination would return an success result
indicating that the pagination token disappeared. After thinking about
it, it's not the best API in the world; ideally, the backpagination
mechanism would restart automatically.

Now, this was handled in the timeline before, and the reason it was
handled there was because it was possible to back-paginate and ask for a
certain number of events. I've removed that feature, so that
back-pagination on a live timeline matches the capabilities of a
focused-timeline back-pagination: one can only ask for a given number of
*events*, not timeline items.

As a matter of fact, this simplifies the code a lot by removing many
data structures, that were also exposed (and unused, since recent
changes) in the FFI layer.