Avoid deadlocks when ensuring Olm sessions for devices
This reworks tracking the Olm sessions a particular task is updating to avoid
deadlocks. By ensuring we synchronously mark all sessions a task cares about as
in progress from the start, we know that no other tasks will own updating a
session in common, which avoids deadlocks across multiple tasks that might be
working on a shared set of devices.
Fixes https://github.com/vector-im/element-web/issues/16194
