Use the tech directly instead of holding references to text tracks in SegmentLoader

[ldayananda]

We currently have this in SegmentLoader, which essentially holds references to text tracks created or referenced by the project:

https://github.com/videojs/http-streaming/blob/21e5335136f714f3783cb5a37d9f06173fbbd9e4/src/segment-loader.js#L171

It would be safer to access the tech directly to access these tracks as they could be removed from the video element by an external project, via the API, e.c.t. We could instead hold onto the ids of the textTracks we created, and use that to request textTracks from the tech.

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