Slow down reconnects on collab (#32418)

We believe that collab deploys currently cause outages because:

* All clients try to reconnect simultaneously
* This causes very high CPU usage on collab (and to some extent, the
database)
* This means that collab is slow to respond to clients
* So clients timeout and retry, over and over again.

We hope by letting clients in in buckets of 512, we can accept some
minor slowness to avoid
complete downtime, while we rewrite the system.

Release Notes:

- N/A

crates/collab/src/tests/test_server.rs

@@ -258,6 +258,7 @@ impl TestServer {
                             None,
                             Some(connection_id_tx),
                             Executor::Deterministic(cx.background_executor().clone()),
+                            None,
                         ))
                         .detach();
                         let connection_id = connection_id_rx.await.map_err(|e| {