Fix RTP ports handling and improve naming of related MediaManager methods

Particularly:
- M4 request needs to use the same ports as M3 response
- M6 response must include a Transport header with both the Source UDP port
and the Sink UDP port

I also tried to split MediaManager into SourceMediaManager and SinkMediaManager, but too many places in the state machine code assume that it is a single class, so I had to abandon the idea. I think eventually we will have to do it, after MediaManager grows too many methods that are either source- or sink- specific.