[proxy] replace the batch cancellation queue, shorten the TTL for cancel keys (#11943)

See #11942 

Idea: 
* if connections are short lived, they can get enqueued and then also
remove themselves later if they never made it to redis. This reduces the
load on the queue.
* short lived connections (<10m, most?) will only issue 1 command, we
remove the delete command and rely on ttl.
* we can enqueue as many commands as we want, as we can always cancel
the enqueue, thanks to the ~~intrusive linked lists~~ `BTreeMap`.

proxy/src/serverless/websocket.rs

@@ -167,7 +167,7 @@ pub(crate) async fn serve_websocket(
         Ok(Some(p)) => {
             ctx.set_success();
             ctx.log_connect();
-            match p.proxy_pass(&config.connect_to_compute).await {
+            match p.proxy_pass().await {
                 Ok(()) => Ok(()),
                 Err(ErrorSource::Client(err)) => Err(err).context("client"),
                 Err(ErrorSource::Compute(err)) => Err(err).context("compute"),