P2P improvement: periodically re-check addresses of seed nodes

[abitmore]

For issue #2153.

Ready for review.

• The first commit contains a code refactory and a small fix ported from Add P2P security options #1764, in order to mitigate potential conflicts when merging Add P2P security options #1764 in the future. Note that it also moves the logging about seed nodes from default to p2p, thus future issues about seed nodes (unable to resolve and etc) will be less visible for node admins.
• The second commit contains code for checking seed nodes updates.