[meta] Improve editoast railjson performance

https://github.com/orgs/osrd-project/projects/12/views/7?pane=issue&itemId=25095490

On a big railjson, the import lasts 3:03 minutes.
1:17 is spent on importing
0:41 is spent on generating

There seems to be a low hanging fruit on: https://github.com/osrd-project/osrd/blob/dev/editoast/src/models/infra.rs#L115-L138 where all the requests are made sequentially. There is a single postgres process at 100% during the import.

If we want to parallelize the import, we need async sql (using something else than diesel?) and we might need to split the transaction (and writing rollback code by hand — but a delete cascade should be quite trivial).

Should we accept to have two SQL connectors (diesel and sqlx ?) ? Or try to switch the whole codebase to https://github.com/weiznich/diesel_async (is would require to also change the connection pool)