stopGracefully
Stops the dispatcher gracefully by shutting down all aggregate dispatchers.
Logs the closure and calls MessageDispatcher.stopGracefully on each aggregate dispatcher. This method waits for all dispatchers to complete their current operations before shutting down. It should be called during the application's shutdown phase.
Return
A Mono that completes when all aggregate dispatchers have stopped gracefully. Completes with an error if any dispatcher fails to stop.