Packetdumper logging duplicates

[mengelbart]

What did you do?

Similar to #84 the packetdumper interceptors pass pointers to rtp.Header around, which can contain byte slices if they contain extensions. These slices can be overwritten by the next incoming packet before the first one was dumped. In that case, the same (later) header extension is dumped twice. I think we can either make a deep copy of the header or format before passing the pointer on to the writer loop. I am not sure which one is more efficient.