recovery: event handlers ran a request in the same task, slowing down the task's progress

[bnjbvr]

In these two event handlers added automatically when enabling key recovery, we start a network request to get the latest version of the state event. This slows down progress on the sync task, which can be an issue if the server is also busy doing other things, or very slow to respond.

Possible fixes:

• run the request in a background task (make sure there's only one, if there's one active consider cancelling it and restarting it, etc.)
• get the event from the store; apparently not the best, because the sliding sync proxy can return stale state account data events.