Add RTP packet pacer
Experiments showed that some network links are sensitive to transmission bursts which cause packet losses. Currently, encoded media (audio/video) are sent on the network as soon as they are delivered by the encoders. For instance, when a video encoder delivers key-frames, the RTP sender will typically receive many RTP packets representing a single frame, which will cause a transmission burst if sent as they arrive. The purpose of this item is to design and implement a transmission pacer to flatten the bitrate curve in order to reduce packet losses caused by transmission bursts. The pacer will limit the rate at which the RPT packets are sent. The pacer will obviously add some delay but its impact should not be noticeable.