mcp: debounce server change notifications (#717)

Debounce and batch notifications for changes in server feature sets
(adding or removing tools, prompts, resources, or resource templates).

This PR handles server-side change notifications only. We could do the
same for client-side notifications (e.g. roots changed), but at the
moment we don't need to.

Fixes #649.

mcp/shared.go

@@ -394,6 +394,8 @@ func notifySessions[S Session, P Params](sessions []S, method string, params P,
 	if sessions == nil {
 		return
 	}
+	// Notify with the background context, so the messages are sent on the
+	// standalone stream.
 	// TODO: make this timeout configurable, or call handleNotify asynchronously.
 	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 	defer cancel()