update BOLA, now works better alongside other rules

[spiterikevin]

Changes include:

1. BolaRule now uses ThroughputHistory instead of doing its own throughput estimation.
2. Remove BolaAbandonRule - just use AbandonRequestRule.
3. Store bolaState in a Dict rather than in a metric.
4. When choosing BOLA through MediaPlayer API, DroppedFramesRule, SwitchHistoryRule and InsufficientBufferRule are still included.
5. Remove logic that is equivalent to InsufficientBufferRule from BolaRule.
6. BolaRule no longer assumes that it is the only rule making ABR decisions.
7. Improve readability and comments.