[backport][DASH] fix live streams not playing, and duration updates.

[glennguy]

Description

Two changes here:

• Updating the duration: some DASH configurations would never update the duration. Fixes for where the timeshiftbuffer duration changes on refresh (fixes Kodi Kayo addon [Aus] time issue for live sport #1152)
• Recent changes in [backport] [DASHTree] handle new Period better #1178 brought a new variable to keep a sequence number on the current period. This lacked initialization causing strange behaviour

Motivation and context

Fixes #1152 and unreported issue

How has this been tested?

Tested on Kayo AU add-on. Before - no playback. Before #1178 - no stream duration updates. After both are fixed.

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• I have read the Contributing document
• My code follows the Code Guidelines of this project
• My change requires a change to the Wiki documentation
• I have updated the documentation accordingly

[CastagnaIT]

nexus branch?

[glennguy]

Since your PR is very close I thought to not bother with Omega. But I can if you expect it to take a while to review and sort out bugs. I don't mind taking the approach of fixing forward with that one though. I'll let you decide, please say if you think I should PR against Omega too :)

[CastagnaIT]

OK then I'll link the PR and i add your changes to my PR, otherwise its difficult tell if it was included in Omega
chenged idea better make separate PR less confusion

[CastagnaIT]

confirmed this has fixed also facebook live regression