Poor connectivity can lead to incomplete sync

### Steps to reproduce

0. I closed my laptop around 01:00
1. I changed my nick on EI around 07:00
2. I opened desktop a while late, around 10:00
3. It tried to incremental sync for *ages* for an hour or so (while on somewhat patchy connectivity)
4. I restarted it at to try to kick it into life at some point
5. Eventually it started syncing again and I can send msgs... but i have ~12 hours of history missing in all(?) of my rooms.

### Outcome

#### What did you expect?

We should never lose history.

#### What happened instead?

We lost history.

### Operating system

macOS

### Application version

Nightly

### How did you install the app?

Nightly

### Homeserver

matrix.org

### Will you send logs?

Yes