Add DBOperator.RunTransaction.
Add DBOperator.Metadata.
c.Check -> c.Assert.
Add DBOperator.TxnOp.
Add DBOperator.BlobStorage.
Add DBOperator.AllMetadata.
Add DBOperator.RemoveMetadata (and Target -> metaColl).
Adjust some error messages.
EnvUUID -> envUUID.
DBOperator -> BackupDB.
managed -> storeImpl.
Leave a TODO for merging with storage.go.