Fix database locked errors (#3153)

* Make read-only operations use WithReadTxn
* Allow one database write thread
* Add unit test for concurrent transactions
* Perform some actions after commit to release txn
* Suppress some errors from cancelled context

internal/manager/task_generate_screenshot.go

@@ -91,7 +91,7 @@ func (t *GenerateScreenshotTask) Start(ctx context.Context) {
 		}
 
 		return nil
-	}); err != nil {
+	}); err != nil && ctx.Err() == nil {
 		logger.Error(err.Error())
 	}
 }