Fetch scrollback history independently of main loop

[ulyssa]

Fetching scrollback history can sometimes take a long time for newly joined rooms, especially some of those on the matrix.org homeserver. This causes the main loop to hang, and prevents the user from being able to navigate away from the room to avoid triggering successive fetches. iamb should use tokio::spawn to do this independently of the main loop, and avoid holding onto the store while doing so.