This is a *subclass* of OWSChatConnectionUsingLibSignal, mainly
because the authenticated/identified connection really does have
capabilities the unauth/unidentified one does not, and splitting those
out makes it easier to see which are which.
For now each websocket connection, identified and unidentified, has
its own libsignal ChatService instance, even though ChatService is
designed to manage both connections together. Later on in the
migration it'll make sense to consolidate them, but under the existing
structure of OWSChatConnection this is the easiest way to manage their
lifecycles.
Controlled by new RemoteConfig flag
"ios.experimentalTransportEnabled.libsignalAuth" (and for local
testing, UserDefaults flag UseLibsignalForIdentifiedWebsocket).
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