improve DHT maintanence performance (by pinging instead of full lookups)

git-svn-id: svn://svn.code.sf.net/p/libtorrent/code/branches/RC_1_0@10452 a83610d8-ad2a-0410-a6ab-fc0612d85776
use get_peers instead of find_node for DHT maintanence. make the dht_routing_table debug output a bit prettier

git-svn-id: svn://svn.code.sf.net/p/libtorrent/code/branches/RC_1_0@10454 a83610d8-ad2a-0410-a6ab-fc0612d85776