Initialse a driver for client and store for future reference
    
    @param client_id: ID of client user
    @return whebwhatsapi object

def init_client(client_id):
    """Initialse a driver for client and store for future reference
    
    @param client_id: ID of client user
    @return whebwhatsapi object
    """
    if client_id not in drivers:
        drivers[client_id] = init_driver(client_id)
    return drivers[client_id]