[sqlite] Mitigate the database locked problem for skylet config (skypilot-org#1509)

* Reorder conn.commit and cursor.close

* WAL mode

* linting

Author: Michaelvll

sky/skylet/configs.py

@@ -13,6 +13,9 @@
 def _safe_cursor():
     """A newly created, auto-commiting, auto-closing cursor."""
     conn = sqlite3.connect(_DB_PATH)
+    # Use WAL mode to avoid locking problem in #1507.
+    # Reference: https://stackoverflow.com/a/39265148
+    conn.execute('PRAGMA journal_mode=WAL')
     cursor = conn.cursor()
     try:
         yield cursor