Use BDB XML transactions for data integrity

[cmlenz]

The BDB XML backend for report data storage doesn't make use of the transactional features provided by the underlying Berkeley DB. However, to reduce the risk of broken data integrity, it should be possible to rollback any changes to the DB when other operations (such as updating the SQLite database) fail.

Note that this still doesn't guarantee data integrity, as there are two storage backends involved in transactions (SQLite and BDB XML), and commiting to the second when the first is already commited may fail, leaving orphaned data in the first backend. This is not critical as long as changes to BDB XML are always commited last, though.

[cmlenz]

Implemented in [209].