OSD: attempts to keep closed files open no longer result in assertion errors...

OSD: attempts to keep closed files open no longer result in assertion errors (this may happen because of delays during the replica reset phase)

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@3143 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534