R1:15dbf6bdf0a5

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…