added auto-sequential feature

git-svn-id: svn://svn.code.sf.net/p/libtorrent/code/trunk@10323 a83610d8-ad2a-0410-a6ab-fc0612d85776
optimize num_seeds() and num_downloaders() to not count the peers every time

git-svn-id: svn://svn.code.sf.net/p/libtorrent/code/trunk@10324 a83610d8-ad2a-0410-a6ab-fc0612d85776