Fix handling of segments at the discontinuities that do not start with a keyframe

[robwalch]

This PR will...

• Prevent backtracking on first segment in discontinuity
• Use gap skipping to handle gap jumping prevent loop loading when large gaps are present in segment/disco start
  • Marks bad segments as GAP segments to avoid loop loading
  • Clears the gap property if the fragment loader later needs to reload the fragment after the buffer was cleared and the fragment had no 'GAP' tag.

Why is this Pull Request needed?

Streams with segments at discontinuity sequence starts that are missing keyframes result in large gaps. HLS.js should not loop load when encountering this scenario.

Are there any points in the code the reviewer needs to double check?

Resolves issues:

Fixes #5629

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md

[robwalch]

@mattzucker,

Let me know if this resolves #5629 for you.