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.
After a resume from sleep, we don't want to start paginating through the whole
timeline. If we were in 'stuckAtBottom' mode before, treat a gappy sync
similarly to how we handle the 'scroll to bottom' knob, which is to reload the
timelinewindow starting from scratch.
Fixes https://github.com/vector-im/vector-web/issues/932, and will probably
also fix https://github.com/vector-im/vector-web/issues/943, though I haven't
tested it yet.
