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_interactive_heatmap_speed.go

@@ -46,10 +46,9 @@ func (t *GenerateInteractiveHeatmapSpeedTask) Start(ctx context.Context) {
 		primaryFile.InteractiveSpeed = &median
 		qb := t.TxnManager.File
 		return qb.Update(ctx, primaryFile)
-	}); err != nil {
+	}); err != nil && ctx.Err() == nil {
 		logger.Error(err.Error())
 	}
-
 }
 
 func (t *GenerateInteractiveHeatmapSpeedTask) shouldGenerate() bool {