honor pieces with priority 7 in sequential download mode

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9868 a83610d8-ad2a-0410-a6ab-fc0612d85776
merged export fix from RC_0_16

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9871 a83610d8-ad2a-0410-a6ab-fc0612d85776