R1:2b473558c2b3

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