server: Minor changes in R/W replication:

- Fixed NullPointer exception in failed() (callback is not always given).
- Decreased logging level of backup replica reset to DEBUG.
- Added extra log message for caught exceptions in processLeaseStateChanged().

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@3226 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534