libxtreemfs: modified execute_sync_request to handle all thread interruption...

libxtreemfs: modified execute_sync_request to handle all thread interruption points + changes according to review on r3004.

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@3019 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534