Need pacing during RTP retransmissions

Context

• A peer has a poor downlink and sends a single NACK packet requesting retransmission of 12 video packets.
• mediasoup NACK generator obtains all those packets (if stored) and sends them all in a row to the poor peer.
• Results cannot be good for obvious reasons, since that burst of RTX packets is gonna affect the poor downlink of the peer even more.

NOTE: When I say "we need pacing" I don't mean a super complex class such as the Pacer of libwebrtc. It could be something simpler such as not retransmitting more than N packets together and delay the retransmissions of the others. We may need a new timer etc.