Sending many events in parallel creates forward extremities instead of linearizing creation

### Description

When sending many events in parallel, Synapse doesn't linearize the event persistence properly, leading to the creation of forward extremities.

I think this is a regression because I recall linearization being fixed years ago, but I don't remember the exact details of the previous fix and couldn't find any obvious problems by scanning recent commits.

### Steps to reproduce

- send multiple events to a single room in parallel (using the standard `/_matrix/client/v3/rooms/.../send/m.room.message/txnid` endpoint, on a single server with a single account)
- observe rows in the `event_forward_extremities` table for the room and the `prev_events` for each created event

Expected result: there is one forward extremity (the last event) and each event references the previous one

Actual result: each event sent in parallel is listed as a forward extremity, and they all reference the same prev event

### Homeserver

maunium.net, localhost, rory.gay, codestorm.net

### Synapse Version

1.129.0

### Database

Tested with PostgreSQL v17 and v14

### Workers

Reproducible with any configuration (single process, one event stream worker, multiple event stream workers)

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Keywords: DAG self-fork