Add built-in seeds for .onion

This makes it possible for a node with `-onlynet=tor` to bootstrap itself. It also adds the base infrastructure for adding IPv6 seed nodes.

The seeds are taken from reliable nodes in @sipa's crawler as well as https://en.bitcoin.it/wiki/Fallback_Nodes#Tor_nodes.

I've tested this with `bitcoind -onlynet=tor -dnsseed=0 -onion=127.0.0.1:9050` after making sure that peers.dat was cleared.