client: Encapsulate xcap renewal state in separate class (identified two...

client: Encapsulate xcap renewal state in separate class (identified two unprotected accesses). Loosen external contract for periodic and blocking tasks.

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@3498 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534