Start the Bokeh Server application.

        Starting the Bokeh Server Tornado application will run periodic
        callbacks for stats logging, cleanup, pinging, etc. Additionally, any
        startup hooks defined by the configured Bokeh applications will be run.

def start(self):
        ''' Start the Bokeh Server application.

        Starting the Bokeh Server Tornado application will run periodic
        callbacks for stats logging, cleanup, pinging, etc. Additionally, any
        startup hooks defined by the configured Bokeh applications will be run.

        '''
        self._stats_job.start()
        if self._mem_job is not None:
            self._mem_job.start()
        self._cleanup_job.start()
        if self._ping_job is not None:
            self._ping_job.start()

        for context in self._applications.values():
            context.run_load_hook()