(stable) Refuse to acquire a second GlobalLock on a connection
Summary:
Ref T13627. MySQL versions older than 5.7 release held locks when a new lock is acquired.
Prevent acquisition of a second lock to prevent this.
Test Plan: Added a failing unit test, made it pass.
Maniphest Tasks: T13627
Differential Revision: https://secure.phabricator.com/D21586