add default_server_name, change spawn start

jupyterhub/utils.py

@@ -214,3 +214,18 @@ def url_path_join(*pieces):
         result = '/'
 
     return result
+
+
+def default_server_name(user):
+    """Return the default name for a new server for a given user.
+
+    Will be the first available integer string, e.g. '1' or '2'.
+    """
+    existing_names = { server.name for server in user.servers }
+    # if there are 5 servers, count from 1 to 6
+    for n in range(1, len(existing_names) + 2):
+        name = str(n)
+        if name not in existing_names:
+            return name
+    raise RuntimeError("It should be impossible to get here")
+