enable TCP keepalive for socks5 connection for UDP associate

git-svn-id: svn://svn.code.sf.net/p/libtorrent/code@10325 a83610d8-ad2a-0410-a6ab-fc0612d85776
merged socks5 keep-alive fix from RC_0_16

git-svn-id: svn://svn.code.sf.net/p/libtorrent/code@10326 a83610d8-ad2a-0410-a6ab-fc0612d85776
update to head of RC_0_16

git-svn-id: svn://svn.code.sf.net/p/libtorrent/code@10327 a83610d8-ad2a-0410-a6ab-fc0612d85776