Abort requests we have no hope of completing in time

[imbcmdth]

We have progress events coming from mediaSegmentRequest that is quite handy. We should leverage that data in that event to decide when to abort a segment request and trigger a rendition switch.

Acceptance Criteria

• Implement the feature
• Test it using bandwidth simulator
• Do a managed release

How

• Improves ABR -
  • Start up time should improve in low bandwidth scenarios
  • Ability to respond to catastrophic bandwidth drops should improve
    • Should lower rebuffering stats (time and count)
• Listen for progress events and judge the ability for the player to complete the request in a reasonable time
• We currently have a hard-timeout. That should be unnecessary in this new world. It might make sense to keep it in the event that progress events are not sent or if they don't contain the kind of data that would allow us to make decisions.
• We need to update the bandwidth property and trigger the bandwidthchange if we decide to abort because otherwise playlist selection doesn't run

Notes

• Bandwidth starts off REALLY low on requests
• The bandwidth simulator DOES simulate progress events (you are welcome)

[mjneil]

this was added in #1176