Fix a race condition handling gappy syncs
We had a problem handling gappy syncs: resetting the timeline would trigger a
pagination request (which would return no results, because there are no events
at this point); this would make the pagination requests which are spawned when
we process the events in the sync get ignored - with the result that we get a
blank window.
The fix is to avoid the ScrollPanel when we are processing new live events and
tell the TimelineWindow to paginate itself directly.
