engine: resources, graph: Change the done channel into a ctx

This is part one of porting Watch to context.

engine/resources/hostname.go

@@ -135,7 +135,7 @@ func (obj *HostnameRes) Watch() error {
 		case <-signals:
 			send = true
 
-		case <-obj.init.Done: // closed by the engine to signal shutdown
+		case <-obj.init.DoneCtx.Done(): // closed by the engine to signal shutdown
 			return nil
 		}