Fix current quality reporting when seeking back from edge in Low-Latency streams

[robwalch]

This PR will...

• Track buffered fragments in Low-Latency HLS so that currentLevel, LEVEL_SWITCHED and FRAG_CHANGED fire when seeking back from edge in Low-Latency streams
• Move appendedPTS from Fragment to fragment-tracker's FragmentEntity

Why is this Pull Request needed?

When streaming Low-latency parts, the fragment-tracker uses activeParts to determine the currentLevel and fragment/level change events. FragmentEntity objects are not stored in the tracker when parts are being loaded as this can interfere with fragment selection and subsequent part loading.

This change stores the last active mainFragEntity FragmentEntity into the fargment-tracker's list of fragments so that when seeking back, getBufferedFrag([at position]) can return the fragment (and its level) for part(s) appended in that range of the buffer.

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

There could be overlap of archived fragments and the archived fragment in certain cases might not match all parts loaded within the buffered range, but I chose not to archive parts because of the increased memory footprint that would add, and the additional work that would be required to make getAppendedFrag and getBufferedFrag find a match based on parts.

Related issues:

• Fix consistency of FRAG_CHANGED and LEVEL_SWITCHED events #5085
• FRAG_CHANGED event not firing consistently #5077
• How to reliably detect current quality level #5082

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]

@erankor This PR may be relevant to you.