Throttle final playlist reloads when using DASH

[gesinger]

Description

HLS already throttles reloads of the final playlist, however, DASH does not. This adds the throttling behavior to the DASH playlist loader.

To see the behavior:

1. Go to https://videojs.github.io/http-streaming/
2. Enter the source as: https://dash.akamaized.net/akamai/bbb_30fps/bbb_30fps.mpd and the mime type as: application/dash+xml
3. Click play
4. Open the console and go to the network tab
5. Click "Offline"

The failed requests will shoot up as fast as can be processed (since the retries are immediate).

The fix makes it so that retries are only made at half the target duration.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors

[ldayananda]

Looks good overall. I think it might make sense to lift this logic to MPC since isFinalRendition is defined there too. But, that can be done in a follow up PR.