Add a very basic "Realtime" log to DarkConsole

Summary: Ref T12568. This begins building toward a more useful realtime debugging console for Leader/Aphlict/general realtime stuff.

Test Plan: {F4911521}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12568

Differential Revision: https://secure.phabricator.com/D17701