Since this already affects more than just the chat connection, having
it in OWSChatConnection isn't really the best choice. This way we can
be sure the libsignal Net instance's proxy settings will be updated
before any notifications go out.
Invoke this non-recommended mode with URL username syntax of
"UNENCRYPTED_FOR_TESTING@my-proxy.net" (or more likely
"UNENCRYPTED_FOR_TESTING@my-dev-machine.local"). The connection to the
Signal servers is still secured by TLS; it's only the outer connection
from the client to the proxy that's now unauthenticated.
