Improve handlers performance

**Feature request**
It would be great to have notifications for middleware.

**Use case**
Many users need this functionality for production deployments.

**Implementation idea**
Add a new config option `enable_notifications` with default `false`.

No file, test, or entry point is named. Start by locating the middleware handlers and configuration parsing, then determine what notifications should contain and how enable_notifications behaves with its default of false. Done means the option is recognized, middleware notifications work when enabled, and the behavior is covered by tests.

Written by the indexing model from the issue text.