Transactional DB interface

This branch adds a transactional interface to the DB to reduce boilerplate. ForumManagerImpl is modified to use the new interface as a proof of concept.

As well as saving some typing, this interface will make it more convenient to expose transactions via the core API, allowing callers to call a series of core methods in a single transaction. This should be more efficient and will also avoid rare bugs where the data changes between one call and the next.