Publish_done/play_done callbacks

Recently I have added a couple of new HTTP callbacks: on_publish_done and on_play_done. The old on_done is also supported. Moreover a bug with calling it twice is now fixed.

Note however these two are just plain notifications. Their result is never analyzed for future unlike on_publish/on_play.

application foo {
    live on;

    on_publish http://example.com/publish;
    on_publish_done http://example.com/publish_done;

    on_play http://example.com/play;
    on_play_done http://example.com/play_done;
}