Split RDBMSManager ClassAdder into separate class(es) to modularise schema management

[andyjefferson]

The RDBMSManager schema management process is embodied in the "ClassAdder" process. This operates in its own transaction using a separate connection. The structure of RDBMSManager is overly complex as a result. We should simply provide a separate process giving it access to the key schema information, and have methods to do specific things

1. add class/classes to persistence
2. delete class from persistence
3. update a mapping in the datstore with new columns e.g when a new implementation of an interface is found.