(stable) Never return external connections to the GlobalLock connection pool

Summary:
Ref T13627. If a lock fails, the connection may be returned to the pool, even if the connection is an external connection. Under old versions of MySQL, connection reuse can release other locks on the same connection.

Don't return external connections to the pool.

This issue was introduced in D21369.

Test Plan: Added a failing test and made it pass.

Maniphest Tasks: T13627

Differential Revision: https://secure.phabricator.com/D21585