Add Low Latency Stall Threshold

[ryanmccartney]

In some cases when using Dash.js in Low Latency mode - seen in this caue with chunks of 0.96s in duration, BUFFER_EMPTY events are not raised when the player is observed to stall.

Current operation has a catch for low-latency mode in the checkIfSufficientBuffer within the BufferController which says if the player is in low latency mode only when the buffer reaches 0 is a BUFFER_EMPTY raised.

In some low latency use cases the player can stall in low-latency mode with a buffer level greater than 0. Particulary when chunk sizes are greater than just a handful of frames.

Outside of low-latency mode the BUFFER_EMPTY event is raised whenever the buffer is below the stallThreshold setting. Set by default to 0.3s.

This PR solves the above by adding a seperate lowLatencyStallThreshold alongside the exisiting stallThreshold to enable users to configure each indepedently to allow for cases when the player is to be used to play both low-latency and regular live DASH content.

Also included are extensions to the reference player samples to allow the configuration of both stallThreshold and lowLatencyStallThreshold