client: do not start xcap renewal if one is already in progress. Fixes...

client: do not start xcap renewal if one is already in progress. Fixes potential crash if second renewal returns and the file handle has been closed in the meantime. 

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@3625 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534