Aphlict - fix getActiveListenerCount return value

Summary: Ref T7126. Dictionaries do not have a length property unlike arrays resulting in the getActiveListenerCount() function returning undefined results. Using the length property on the array of keys will work.

Test Plan: Using wscat to generate multiple connections to the server, establish new ones and close others while keeping an eye on the displayed clients.active value.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7126

Differential Revision: https://secure.phabricator.com/D11647