rest-server/daemon: use sd_notify_barrier for service reloading

until now, we manually polled the systemd service state during a reload
so that the sd_notify messages get processed in the correct order
(RELOAD(old) -> MAINPID(old) -> READY(new))

with systemd >= 246 there is now 'sd_notify_barrier' which
blocks until systemd processed all prior messages

with that change, the daemon does not need to know the service name anymore

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

proxmox-rest-server/examples/minimal-rest-server.rs

@@ -213,7 +213,6 @@ async fn run() -> Result<(), Error> {
                 Ok(())
             })
         },
-        "example_server",
     ).await?;
 
     Ok(())