Improve read marker behaviour

[bwindels]

On riot-web, we've updated the behaviour of the read marker to address https://github.com/vector-im/riot-web/issues/4134. It would be great to implement this change across riot on all platforms. Like before, when the read marker is moved, it gets set to after the last message (partially) in the viewport.

• when the event for the read marker is currently within the viewport, the read marker gets moved after 1 second of user activity*.
• when the event for the read marker is currently above the viewport, the read marker gets moved after 30 seconds of user activity*. This isn't ideal though, and will be changed to set a flag instead and only update the read marker when moving away from the room.
• for clarity: (when the event for the read marker is currently below the viewport, the read marker is untouched, as before).

Scrolling the viewport should update the above evaluations (e.g. cut the out-of-viewport timer, and start the in-viewport one, when scrolling the current read marker event in view).

* The user is considered active up to 2 minutes after having last scrolled, clicked, or having switched to the app. When moving away from the app, the user is immediately marked as inactive.

The actual timeout values may be adjusted in the future.

Implementation details: there is no danger of different clients resetting the RM, as synapse already takes the most recent event between a new and the existing read marker.

riot-web PR: matrix-org/matrix-react-sdk#2345