Fix room change sometimes being very slow
If the js-sdk had a lot of history in memory for a particular room,
riot would paginate all that history into the DOM and render it
when switching to that room (before then removing it all again).
This obviously made switching to that room very slow.
This was caused by the fact that we relied on the setState that
happens in TimelinePanel after the pagination taking effect such
that ScrollPanel sees that it no longer needs to paginate, but
in some situations (as far as I can see, in electron...?) this
setState would not take effect until the pagination stopped
fulfiling requests from memory and hit the network.
Fix: don't resolve the promise returned by the pagination request
until the setState has actually happened.
