Don't poll showmyip.com, it doesn't exist anymore

Fixes #4679.

This leaves us with only one candidate, checkip.dyndns.org.
GetMyExternalIP should be phased out as soon as possible.

Rebased-From: c33b983
Avoid querying DNS seeds, if we have open connections.

The goal is to increase independence and privacy.
Merge pull request #674 from rnicoll/1.8.1-dev-networking

Apply network related changes from 0.9.3