Get client remote address from SDP for ampcache rendezvous

We use the candidate attributes from the client's SDP to make a best guess of the remote address for polls via SQS rendezvous. We can use the same technique for AMP cache polls, which have a similar problem of always being geolocated to the AMP cache server's IP. It can be useful to see which rendezvous methods work best under different censorship regions.

I'd like to slightly refactor this code while we're at it to avoid duplicate decodings of the client poll messages.