introduced a new alert torrent_update_alert, for when a torrent_handle changes info-hash

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@8409 a83610d8-ad2a-0410-a6ab-fc0612d85776
debug invariant checks for DHT observer allocator

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@8410 a83610d8-ad2a-0410-a6ab-fc0612d85776
fix assert

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@8413 a83610d8-ad2a-0410-a6ab-fc0612d85776