tigefa4u projects Trac

* Renamed `IDatabaseBackend` to `IDatabaseConnector`, and `DatabaseBackendManager` to just `DatabaseManager`.
* The scheme prioritization is gone, the algorithm remains; the actual connector implementations with different priorities for different schemes can be easily added back in later... (but let's keep it out for now, please).
* Moved connection pool stuff into a separate module.
* Moved schema classes into separate module.
* Moved utility classes (`ConnectionWrapper` and `IterableCursor`) into a separate module.
* Renamed the modules for PostgreSQL and SQLite support.