Refactor: controller: New controld_globals.cib_client_id member

Store the controller's CIB client ID when we successfully connect to the
CIB. We'll use this in an upcoming commit to check whether we requested
a CIB replacement.

Also make a file-scope variable a static function-scope variable.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>