Message send order has regressed once more when on bad connectivity.

### Steps to reproduce

1. Be on bad connectivity (Eurostar in this case)
2. Queue up some messages
3. Probably stab 'retry' a few times
4. When they finally unqueue, they send in the wrong order.
5. Interestingly(?) they are then shown in the wrong order in the timeline.

Related to https://github.com/element-hq/element-web/issues/5408 and maybe https://github.com/element-hq/element-web/issues/18942 and https://github.com/element-hq/element-web/issues/29677 and many others.

### Outcome

#### What did you expect?

Unqueue the messages in the right order. Don't call /send on the next until the previous /send has got a 200.

#### What happened instead?

Looks like it tried to call /send in parallel and they raced.

### Operating system

15.7.1 (24G231)

### Browser information

145.0.2 (aarch64)

### URL for webapp

develop.element.io

### Application version

Element version: 4987d6c57371-js-942fdf5bee0f Crypto version: Rust SDK 0.14.0 (c3b7918), Vodozemac 0.9.0

### Homeserver

matrix.org

### Will you send logs?

Yes