Closes getlantern/lantern#2065 Added timeout to upnpc call in go-igdman

@myleshorton I've never been able to reproduce this issue, but after reviewing the go-igdman code I did find one call to upnpc that didn't have a timeout, so I fixed that.  Would you mind retesting with this branch on that network where you see the issue?

I also went ahead and added a timeout to determineInternalIP() and added a comment to explain why we're doing what we're doing.