fixed bug causing forum posts to disappear/re-appear and to be set unread again. The bug was due to inconsistency between cleaning old messages and still inserting old messages again. Also augmented the storage period to 4 months (previously 1 month). Challenge: we should keep threads until the newest post in the thread is older than 4 months

git-svn-id: http://svn.code.sf.net/p/retroshare/code/trunk@7554 b45a01b8-16f6-495d-af2f-9b41ad6348cc