Clean up EventSource interface

Remove the 'key' parameter to 'accept', 'read' and 'write' methods.
It is not needed, because an EventSource is expected keep track of
its channel and selection key.

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Merge pull request #42 from jvirtanen/cleanups/event-source-interface

Clean up EventSource interface