Active management of the MSE Source Buffer

[ryanmccartney]

Introduces some additional settings and a flag to control their use.

Provides a mechanism for Dash.js to control the level of the MSE source buffer on startup or recovery from a a stall in order to prevent short stalls.

Many TV devices already appear to take steps to do this in their MSE implementations. Here, we provide a universal mechanism to ensure all devices benefit from a cautious startup after a stall.

New settings:

• loadThreshold - Compliments stallThreshold symmetric by default, but allows a difference recovery threshold to be set than the stall threshold for a more cautious startup.
• lowLatencyLoadThreshold - Low latency equivalent of the above. Compliments lowLatencyStallThreshold symmetric by default but allows a difference recovery threshold to be set than the stall threshold for a more cautious startup.
• activeSourceBufferManagement - Controls whether the above are used to stop playback (by setting rate to 0) in the period of time between BUFFER_EMPTY and BUFFER_LOADED events.