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

git-svn-id: http://libtorrent.googlecode.com/svn@10452 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda
merged chagnes from RC_1_0

git-svn-id: http://libtorrent.googlecode.com/svn@10453 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda
use get_peers instead of find_node for DHT maintanence. make the dht_routing_table debug output a bit prettier

git-svn-id: http://libtorrent.googlecode.com/svn@10454 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda