WIP: rtpjitterbuffer: add DTX functionality
Some audio-codecs (like opus) uses DTX (Discontinuous transmission), in the way that they simply stop sending audio-packets when there is no audio detected.
The expectation is then that the codec will use its internal PLC (packet loss concealment) to create packets in place of those missing.
This patch is reusing the timer already existing for RTX and lost-events, scheduling a lost-timer for the next sequence-number based on the inter-packet spacing.
Edited by Tim-Philipp Müller