refactor: postgres_backend: replace abstract shutdown_watcher with CancellationToken (#8295)

Preliminary refactoring while working on
https://github.com/neondatabase/neon/issues/7427
and specifically https://github.com/neondatabase/neon/pull/8286

safekeeper/src/wal_service.rs

@@ -4,9 +4,10 @@
 //!
 use anyhow::{Context, Result};
 use postgres_backend::QueryError;
-use std::{future, time::Duration};
+use std::time::Duration;
 use tokio::net::TcpStream;
 use tokio_io_timeout::TimeoutReader;
+use tokio_util::sync::CancellationToken;
 use tracing::*;
 use utils::{auth::Scope, measured_stream::MeasuredStream};
 
@@ -100,7 +101,7 @@ async fn handle_socket(
     // libpq protocol between safekeeper and walproposer / pageserver
     // We don't use shutdown.
     pgbackend
-        .run(&mut conn_handler, future::pending::<()>)
+        .run(&mut conn_handler, &CancellationToken::new())
         .await
 }