Reduce serialization errors in MultiWriterIdGen (#8456)

We call `_update_stream_positions_table_txn` a lot, which is an UPSERT
that can conflict in `REPEATABLE READ` isolation level. Instead of doing
a transaction consisting of a single query we may as well run it outside
of a transaction.

tests/storage/test_base.py

@@ -56,6 +56,7 @@ class SQLBaseStoreTestCase(unittest.TestCase):
         engine = create_engine(sqlite_config)
         fake_engine = Mock(wraps=engine)
         fake_engine.can_native_upsert = False
+        fake_engine.in_transaction.return_value = False
 
         db = DatabasePool(Mock(), Mock(config=sqlite_config), fake_engine)
         db._db_pool = self.db_pool