don't block when changing rss feed settings

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@8868 a83610d8-ad2a-0410-a6ab-fc0612d85776
merged fix from RC_0_16

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@8870 a83610d8-ad2a-0410-a6ab-fc0612d85776