VBS/VGCS support in osmo-mgw
In a broadcast call scenario, we need a type of endpoint which has
- one "source" RTP connection (e.g. from upstream MSC, or from individual talker)
- many "destination" RTP connections (one for each BTS in which group members are)
In a group call scenario, we also dynamically change around which is the "source" RTP connection, depending on who is the currenct talker.