two improvements of turtle router logic: increase of TR cache storage time to avoid too many void tunnels due to bouncing TR beyond storage period of 120 secs; prevent openning upload tunnels for files currently being downloaded before any chunk was obtained. Suggested by Jo.

git-svn-id: http://svn.code.sf.net/p/retroshare/code/trunk@7719 b45a01b8-16f6-495d-af2f-9b41ad6348cc