Implement OS signal handling for event stream

See https://github.com/yiisoft/yii-debug-api/blob/23c4bea89ecd8e45038df9f0eab77ca59a4d9033/src/Controller/DebugController.php#L337 (https://github.com/yiisoft/yii-debug-api/pull/117)

Event stream doesn't respect any OS signal, it just takes its time to wait (sleep) and them exits.

