OSD: fixed a bug that caused invalid checksum calculations, which lead to...

OSD: fixed a bug that caused invalid checksum calculations, which lead to missing data in response to read requests when checksums were enabled, added more debug output when reading objects

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@736 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534