When a GlobalLock with an external connection is released, don't return it to the pool

Summary:
Ref T13627. Currently, global locks always return connections (even external connections) to the connection pool when unlocked.

This code is obviously buggy: isExternalConnection is set to false immediately before it is tested. This bug has existed since this code was introduced, in D15792.

• Instead of storing a flag, store the actual connection.
• Don't clear it when unlocking.
• Don't return external connections to the pool.

Test Plan:

• Added a failing test, made it pass.

Maniphest Tasks: T13627

Differential Revision: https://secure.phabricator.com/D21583