A transcode that is not in the store is invisible to the other replicas, so the
HLS entry points have to publish and hold the lease themselves.
- register the play session in the store when ffmpeg starts
- renew the lease on a background loop and delete the session when it ends
- shorten segments and bound the playlist window when resuming a stored session
- add RecoverySegmentLengthSeconds and RecoverySegmentBufferCount encoding options
