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