R1:466013f11a6d

Prevent external connections from being mutated on held locks

Summary: Ref T13627. This makes the API harder to misuse: setting an external connection on a held lock isn't a meaningful operation. Prevent it.

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

Maniphest Tasks: T13627

Differential Revision: https://secure.phabricator.com/D21584