improve DHT routing table to return better nodes (lower RTT and closer to target)

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@8925 a83610d8-ad2a-0410-a6ab-fc0612d85776
restore commented out function

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@8926 a83610d8-ad2a-0410-a6ab-fc0612d85776
abstract generic_category() to build on boost-1.42 and older. fix some include issues in tests

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@8927 a83610d8-ad2a-0410-a6ab-fc0612d85776
fix build with old asio (boost-1.42 era)

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@8928 a83610d8-ad2a-0410-a6ab-fc0612d85776
introduce new, optional, get_peers mechanism that is more privacy preserving (doesn't leak the info-hash to as many DHT nodes)

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@8929 a83610d8-ad2a-0410-a6ab-fc0612d85776