tune: raise start_all concurrency from 3 to 10

3 is too conservative for modern hardware; 10 provides a moderate boost
without the risk of overwhelming the host (start is the heaviest
operation: ubridge process + docker start + network config for each
node).

gns3server/controller/project.py

@@ -2078,7 +2078,7 @@ class Project:
         if not nodes_to_start:
             return
         log.info("Project '%s' [%s]: starting %d nodes...", self._name, self._id, len(nodes_to_start))
-        pool = Pool(concurrency=3)
+        pool = Pool(concurrency=10)
         for node in nodes_to_start:
             pool.append(node.start)
         await pool.join()