Fix: Reduce max buffer length

[robwalch]

This PR will...

Reduce max buffer length in response to buffer full errors using the duration of the fragment that caused the error .

• Max buffer length is reduced by three fragment durations or half (whichever result is greater)
• The minimum value for max buffer length is constrained between maxBufferLength and fragment duration

Why is this Pull Request needed?

reduceMaxBufferLength was a no-op when its input was greater than half maxMaxBufferLength.

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

Resolves issues:

Fixes #6529

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