Replace DVRInfo metric with dvrwindowchanged event and MediaPlayer API

[LloydW93]

Previously, in issue #838, the wallclock interval was decreased due to it increasing startup delay.

However, looking at where this is used today, it is only used to control buffer pruning and updating the availability window - and running this every 50ms is in my opinion excessive.

Meanwhile, the DVRInfo metric is updated on every buffer level change, even when the content is on demand. This means that if you pause playback, the client does not have any events to inform it of a change in the available window. In the case of a UI that draws a control bar for the current availability window such as http://is.gd/ExzwTu, this results in a broken experience for users.

To solve both of these problems in our use case, we increased the wall clock time to 1000, and send the DVRInfo event every tick, for dynamic content only: https://github.com/Dash-Industry-Forum/dash.js/compare/development...bbc:dvr-wallclock?expand=1

To improve this behaviour more generally though, I think we need to consider:

• The value of a global wall clock timer that is now only used in two places, as everywhere else now has its own timers (with intervals that make sense for that specific use).
• Whether using 'metricadded' of type DVRInfo is the best way to get the current availability window (I may be missing an API for this). In an ideal world it'd be possible to set seekable on the videoModel (Seekable differs from non-MSE behavior w3c/media-source#5), but it currently isn't.
• The general situation with metric data slowly increasing resource usage. Dash.js 2.1.1's behaviour is to append the current DVRInfo to an array every 50ms, even for ondemand content. Can we look at truncating the array (and other metric arrays) after, say, 1000 data points?

[dsparacio]

Adding reference to this issue as it may be related #887

[dsparacio]

I am for removing DVR Info as a metric and using event driven data and model store.

[LloydW93]

I think it makes sense to break this into separate issues (as alluded to by the bullet points in the original message):

• Removing the DVRInfo metric, and replacing it with an event such as "dvrwindowchanged" and a getDVRWindow API on MediaPlayer (2.2 milestone), only populating it for dynamic manifests
• The removal of a global wall clock time (put on backlog) - Remove the global wallclock time #1388
• .seekable on the VideoModel (already covered by Consider adding seekable support? #1384)
• Performance improvements for metric storage (2.3 milestone, perhaps?) - Improve efficiency of metric storage #1389

I'll update this issue title to cover the changes to the first part only, and raise new issues for the others.

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