App crashes while restoring online backup

### Steps to reproduce

1. Have an account with a lot of keys (probably 300MB or so)
2. Log in fresh
3. Try to restore online backup
4. Wait 5 minutes, then watch as the app crashes (albeit with no prompt to report a bug to apple or to us)

### Outcome

#### What did you expect?

online backup that restores sessions incrementally so you don't (presumably) run out of memory and crash by trying to do them all at the same time.

#### What happened instead?

online backup which crashes on big accounts.

### Your phone model

iphone 12 pro max

### Operating system version

iOS 15.4

### Application version

1.8.15 tf

### Homeserver

matrix.org

### Will you send logs?

Yes

Start by reproducing online backup restoration with an account containing roughly 300MB of keys on an iPhone 12 Pro Max running iOS 15.4, then inspect the resulting crash logs. Done means large accounts restore incrementally without the app crashing.

Written by the indexing model from the issue text.