Support recovery from lost iTunes audio packets

[watson]

Currently the iTunes client will not respond to retransmission requests on the control port. This works fine with the iOS client though, so I don't know what's wrong.

[skerit]

I think I figured it out: itunes will send the retransmit response to the port that made the original request.
It just ignores the control port you told it earlier to use.

So sending the request via the rtp-control-server socket (the one that receives the responses) should fix this issue. (And that's also one less socket to create for every request, so there must be speed boost in there, too :))