Handle input asset `start_time` on transcode and prepare

[victorges]

When processing an asset through broadcasters, we demux the segments and transcode them independently. We then create an HLS playlist with all the output segments or a file with no metadata coming from the original file.

This causes things like start_time to be ignored, which can make a video have off-sync audio issues. This is my hypothesis based on the little I know and investigated about these files and behavior.

This might also be fixed by #18 at least for the transcode task. We might still need to do something for HLS playlists though, like for example using the new clipping feature livepeer/go-livepeer#2280 on the works!

[pglowacky]

Unclear how often this happens, we assume that this happens most often when users are manipulating the video post production. When this happens, it makes the video unusable.

Could be solved by:

• Clipping
• passing along the metadata.