* Add restore validation feature with simplified backup gap fix
Implements restore validation using audit_storage to verify backup/restore
correctness. Includes a minimal fix for the backup gap bug.
Key components:
- ValidateRestore audit type: compares source keys against restored keys
at \xff\x02/rlog/ prefix in storage server
- DD audit fixes: propagate validation errors, handle DD failover correctly
- RestoreValidation and BackupAndRestoreValidation workloads for testing
- Simplified backup gap fix: prevent snapshot from finishing in the same
iteration it dispatches the last tasks (single flag + one check)
aa35d6cc29