[server] Fixed issue with private threads having the wrong name

Summary: Fixed the issue with private threads having the thread name of the userID instead of the username.

Test Plan: Tested in native and phpMyAdmin to make sure the correct thread name was set.

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D713