iptv: new features for multicast, rtsp & rtcp

iptv: new features for multicast, rtsp & rtcp

- Implement RTCP Negative Acknowledge (a.k.a. Retransmission) support for RTP streams.
    When packet loss is detected the client will send a RTCP Generic Feedback report to the server. The server can than resend these lost packets.
    Retransmitted packets are send through a second connection or as part of the main stream, both cases are supported.
    For Multicast manual setup of the RTCP server is required, for RTSP automatic setup (was already implemented for Receiver Reports) or manual override is possible.
- General clean-up of unused RTCP code and restructure to allow for easy implementation of different types of RTCP messages.
- Make RTCP Receiver Reports optional.
- RTSP start session with DESCRIBE and parse response content.
- RTSP DESCRIBE redirect support.
- Parse DESCRIBE response for AVPF support (required for Retransmission).
- Implement remote time shift support for RTSP streams.
    This option can be enabled for a channel to pass-through time shift commands to the RTSP server, the internal time shift buffer is then disabled.