update AbandonRequestsRule - improved throughput estimation, now minds buffer level

Created by: spiterikevin

Improve throughput estimation: AbandonRequestsRule was not reacting in time if there is a sudden drop in bandwidth, sometimes allowing avoidable rebuffering. The throughput estimate maintained by the rule was too slow to react. This update maintains a more accurate throughput estimate.

Introduce buffer level consideration: This updated rule is more conservative (more likely to abandon) if the buffer level is very low. In addition to comparing estimateTimeOfDownload with the segmentDuration * 1.8, the updated rule now also compares it with bufferLevel.