rtpmanager: improve performance in NACK/RTX handling

Submitted by Miguel París Díaz @mparisdiaz

Link to original bug (#789338)

Description

Hello,
I am providing several changes to improve NACK/RTX handling.
The general remarks are:

• Add GstRTPRetransmissionListRequest that allows send a list of seqnum to ask for NACKS. avoiding sending a lot of events on packet looses bursts.
• GstRtpRtxQueue: several changes to avoid unnecessary operations.
• Add "rtt" field in both GstRTPRetransmissionRequest and GstRTPRetransmissionListRequest that allows upstream elements like GstRtpRtxQueue to send retransmissions under the RTT.

Anyway, each commit has more detailed info.

Version: 1.8.3