Fix appending of Interstitials in place that exceed X-PLAYOUT-LIMIT

[robwalch]

This PR will...

Limit append-in-place buffering to playout-limit.

Why is this Pull Request needed?

Buffering of interstitial assets was not stopped when the forward buffer past X-PLAYOUT-LIMIT. This is expected when each asset player resets the media source and X-PLAYOUT-LIMIT is enforced by monitoring currentTime.

When all assets are appended in place, sharing a single MediaSource, buffering must stop after reaching or exceeding the playout limit, so that the MediaSource can be transferred to the hls instance responsible for buffering the next scheduled segment.

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

Resolves issues:

Fixes #7179

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

[martmol]

Tested this PR and it's fixing (partially) the bug (When not on livepoint) 🚀 Two things;

• I noticed that sometimes the interstitial continues for a bit (up to couple of seconds) after INTERSTITIAL_ENDED was triggered, probably playing the last buffered bit.
• When on live point, INTERSTITIAL_ENDED is triggered but interstitial continues to play. Is this because the X-PLAYOUT-LIMIT isn't known when the interstitial starts playing?