upid: use systemd escape to decode/encode the worker_id

This way we can store values containing "/" and ":".

src/api2/node/tasks.rs

@@ -342,7 +342,7 @@ pub fn list_tasks(
             if info.upid.worker_type == "backup" || info.upid.worker_type == "restore" ||
                 info.upid.worker_type == "prune"
             {
-                let prefix = format!("{}_", store);
+                let prefix = format!("{}:", store);
                 if !worker_id.starts_with(&prefix) { return None; }
             } else if info.upid.worker_type == "garbage_collection" {
                 if worker_id != store { return None; }