It turns out that a bunch of things rely on MatrixClient methods to return
promises rather than undefined. Rather than having to undo half the work done
by sinon.createStubInstance, just build our own object with as many methods as
we need stubbed out.
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.
