DELETE /_synapse/admin/v1/rooms/<room_id> should be an async endpoint

[Twi1ightSparkle]

https://matrix-org.github.io/synapse/latest/admin_api/rooms.html#delete-room-api

Sometimes it takes while to delete rooms resulting in HTTP timeouts. This endpoint should be made async to mitigate this.

[clokep]

We should do something similar to the purge history API where it returns an ID of some sort and you can then use another API to request a status update on that API, see https://matrix-org.github.io/synapse/latest/admin_api/purge_history_api.html

[dklimpel]

It is a dup to #10797

[clokep]

Good catch!