fix bug in udp_socket when changing socks5 proxy quickly

git-svn-id: http://libtorrent.googlecode.com/svn/trunk@10056 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda
update changelog now that the udp_socket fix is back-ported to RC_0_16

git-svn-id: http://libtorrent.googlecode.com/svn/trunk@10058 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda