Also cleaned up a few unused #if blocks, and started sending a few ERROR
events back to the apps.
Resolves: #1574
DeveloperNote: Apps can now send a `MARK_MEDIA` command with an optional
`correlation_id` parameter to chan_websocket which will be placed in the
media frame queue. When that frame is dequeued after all intervening media
has been played to the core, chan_websocket will send a
`MEDIA_MARK_PROCESSED` event to the app with the same correlation_id
(if any).
4fa314242b