Interstitial fragment loading and buffering fixes

[robwalch]

This PR will...

• Fix streaming primary content over queued assets (base-stream-controller)
• Fix "Main forward buffer length" warning on "seeked" for asset players (stream-controller)
• Fix audio streaming gaps in asset players created after buffer is populated (handles incomplete fragment tracker - audio-stream-controller)
• Fix incorrect interstitial buffering state after flush (interstitials-controller)
• Fix advancing interstitial buffering advancement with append-in-place (interstitials-controller)
• Fix interstitial start/end events at edge (playing last schedule item transitions with new interstitial) (interstitials-controller)
• Fix asset player queue leaks (interstitials-controller clearAssetPlayer exits early resulting in playerQueue buildup)
• Allow append-in-place at live edge with single playlist media selection (not allowed with alt-audio) (interstitials-schedule) resolves Interstitial append in place not happening when on live point #6995
• Run skip buffer hole when stalling at a buffered range that ends prior to live playlist window start (gap-controller)
• Add safety around asset player getters that throw when destroyed (interstitial-player)

Why is this Pull Request needed?

Improves Interstitials playback and seeking with config.appendInPlace enabled.

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

Resolves issues:

Resolves #6995

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