Make gap controller wait for active fragments while seeking

[robwalch]

This PR will...

Make the gap controller wait for active fragments that start before current position while seeking.

Why is this Pull Request needed?

When seeking to a point with 2 seconds (MAX_START_GAP_JUMP) before a buffered range, the gap-controller will seek to the start of that range without waiting for fragments to be loaded and appended that would fill the buffer. This change makes the gap controller wait so that rather than skipping ahead, it seeks to and plays the media from the main playlist at the initial seek point.

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

Only active fragments from the stream-controller (main playlist) are considered. Alternate audio track fragments are not included in the check. Generally audio segments are smaller and are delivered ahead of video, but this might not always be the case, so passing in both could be a good future enhancement (once the stream controller can monitor the audio stream controller, or the gap controller operates with access to both).

Resolves issues:

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]

@karendolan after merging #4667 I noticed even more that we were executing start gap seeks when seeking to a point before a buffered range. This should address that and give us more precise (although occasionally slower) seeking.

[robwalch]

Related to #2327.