Add 'EventSource#isClosed' back

Reviewed-by: Pekka Enberg <penberg@valotrading.com>
Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Add 'Events#close'

Reviewed-by: Pekka Enberg <penberg@valotrading.com>
Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>