Power level edits can race with the remote echo

### Steps to reproduce

1. Change someone's power level
2. Before that first power levels event comes down the remote echo, change someone else's power level

### Outcome

#### What did you expect?

Either it shouldn't be possible to change the 2nd user's power level while the first request is still in flight, or the 2nd power level event should take the local echo of the 1st power level event into account

#### What happened instead?

The 2nd power level event doesn't include the changes that were made in the 1st power level event, meaning that the 1st change is unexpectedly undone

### Operating system

NixOS unstable

### Browser information

Firefox 100.0

### URL for webapp

develop.element.io

### Application version

Element version: 479d4bf64d97-react-26bc3ab0aa60-js-4721aa1d241a Olm version: 3.2.8

### Homeserver

matrix.org

### Will you send logs?

No