datachannel: Notify low buffered amount according to spec
Quoting https://www.w3.org/TR/webrtc/#dom-rtcdatachannel-bufferedamountlowthreshold
The bufferedAmountLowThreshold attribute sets the threshold at which the bufferedAmount is considered to be low. When the bufferedAmount decreases from above this threshold to equal or below it, the bufferedamountlow event fires.