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_phash.go

@@ -44,8 +44,8 @@ func (t *GeneratePhashTask) Start(ctx context.Context) {
 		})
 
 		return qb.Update(ctx, t.File)
-	}); err != nil {
-		logger.Error(err.Error())
+	}); err != nil && ctx.Err() == nil {
+		logger.Errorf("Error setting phash: %v", err)
 	}
 }