Refactored OperationBackupHandler in BasicOperationService as a singleton instance instead of creating a new instance of it on every backup operation.
Cleaned up BackupTest to reduce execution time.
Set "initialized" flag of partition service before processing partition table. Fixes #2804
Merge pull request #2908 from mdogan/fix/3.3/issue-2804

Fixes data loss issue when  partition table is being synced and a node gracefully shutdown