minor performance optimization of udp trackers

git-svn-id: http://libtorrent.googlecode.com/svn@10398 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda
merged changes from RC_1_0

git-svn-id: http://libtorrent.googlecode.com/svn@10399 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda