WIP: proper syncing for non-muxed HLS live feeds

[squarebracket]

Description

Corrects a/v desync for non-muxed live feeds by properly syncing the timestampOffset of segment loaders when corresponding segments have different starting DTSs. Note that, since VODs don't call timestampOffset, this only affects live feeds.

I get sporadic sync problems so I'll still need to look into that.

Specific Changes proposed

• Use the timingInfo of the first finished segment to dispatch a firststarttime event to the master playlist controller
• Set the dtsSync_ of the MPC if it's null (from the event data), then dispatch a dtsSync to the calling segment loader with the received start and this.dtsSync_.
• The dtsSync event handler on the segment loader sets the timestamp offset from the reference

Requirements Checklist

• [ ] Feature implemented / Bug fixed
• [ ] If necessary, more likely in a feature request than a bug fix
  • [ ] Unit Tests updated or fixed
  • [ ] Docs/guides updated
  • [ ] Example created (starter template on JSBin)
• [ ] Reviewed by Two Core Contributors

[forbesjo]

Thank you for your PR but we will not be accepting new changes for this repo and will be archived very soon. I would advise you to open your PR against the next iteration of this project at https://github.com/videojs/http-streaming.