Decrease JX.Leader lease duration from 16,000ms to 1,500ms and usurp more aggressively

Summary:
Ref T12573. JX.Leader synchronizes the Aphlict connection across multiple windows.

Currently, we only test to see if the leader window has been closed every 16 seconds. Instead, test every 1.5 seconds.

Also, make windows keep trying to become the leader forever. This was removed previously (in D15806) but I think that change decreased robustness here.

Test Plan:

• Opened two windows to the "Realtime" tab in DarkConsole.
• Saw one become the leader and one become a follower.
• (Optionally, wait for 10 seconds here to test the "keep trying to become the leader" behavior.)
• Closed the leader.
• Saw the follower become the leader after ~1.5 seconds.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12573

Differential Revision: https://secure.phabricator.com/D17703