Optimize handling of closed connections

Instead of iterating through all event sources after each and every
'Selector#select' or, in particular, 'Selector#selectNow', notice a
connection getting closed via 'ReadableChannel#read' returning -1.

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Remove 'EventSource#unregister'

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>