server: OSD: HashStorageLayout:

- Backported Lukas' fix which re-enables checking the stored checksum.
- Added functionality to retry reading the data in case of an incomplete read as it may be caused by a bad sector. While retrying the read, the disk firmware might remap the sector / recover the data.

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@3506 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534