To support modularization (eg of isisaddons), it'd be good practice to allow classes to be mapped to specific (DB) schemas, rather than the default "public" (HSQLDB/Postgres) or "dbo" (MSSQL) schema.
This requires some mechanism to intercept the creation process and create the schema before hand.
This enhancement is:
- provide the ability to register a listener class (implementing org.datanucleus.metadata.MetaDataListener), so that this can be done on start-up
- provide a default implementation that works "out-of-the-box" for (at least) HSQLDB, PostgreSQL and MS SQL Server.