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.
Source side is using SetSinkRtpPorts(), SinkRtpPorts() and SourceRtpPort(). Sink side is using only SinkRtpPorts().

I agree it's a good idea to split and I tried, but I don't know how to do it cleanly. The approach should be decided by someone who wrote the code, and understands the design better. Specifically, the problem is in MessageHandler class, which is the base class for message handling on both source and sink sides, and which contains the MediaManager instance as a member (it is used by subclasses on both source and sink sides). How to redesign that part? @pozdnyakov ?