token Server Url Override
URL for the user's Sync Tokenserver. This can be used to support users who self-host their sync data. If None then it will default to the Mozilla-hosted Sync server.
Note: this lives here for historical reasons, but probably shouldn't. Applications pass the token server URL they get from fxa-client to SyncManager. It would be simpler to cut out fxa-client out of the middle and have applications send the overridden URL directly to SyncManager.