Proxy: send cancel notifications to all instances (#6719)

## Problem

If cancel request ends up on the wrong proxy instance, it doesn't take
an effect.

## Summary of changes

Send redis notifications to all proxy pods about the cancel request.

Related issue: https://github.com/neondatabase/neon/issues/5839,
https://github.com/neondatabase/cloud/issues/10262

proxy/src/serverless/websocket.rs

@@ -1,5 +1,5 @@
 use crate::{
-    cancellation::CancelMap,
+    cancellation::CancellationHandler,
     config::ProxyConfig,
     context::RequestMonitoring,
     error::{io_error, ReportableError},
@@ -133,7 +133,7 @@ pub async fn serve_websocket(
     config: &'static ProxyConfig,
     mut ctx: RequestMonitoring,
     websocket: HyperWebsocket,
-    cancel_map: Arc<CancelMap>,
+    cancellation_handler: Arc<CancellationHandler>,
     hostname: Option<String>,
     endpoint_rate_limiter: Arc<EndpointRateLimiter>,
 ) -> anyhow::Result<()> {
@@ -141,7 +141,7 @@ pub async fn serve_websocket(
     let res = handle_client(
         config,
         &mut ctx,
-        cancel_map,
+        cancellation_handler,
         WebSocketRw::new(websocket),
         ClientMode::Websockets { hostname },
         endpoint_rate_limiter,