Under certain conditions, it was possible to get stuck in a state where any
user-initiated scroll would be met with "Working around
vector-im/vector-web#528" and overridden. Fix this by removing the duplication
between _lastSetScroll and recentEventScroll, and using _lastSetScroll which is
more reliable.
I think this was responsible for the readmarker just disappearing rather than
animating.
While we're here, decrease the delay on the animation, and instead make it take
slightly longer to disappear.