enable debug logging
refactoring (Default)AdministrationDao to be split into two parts and don't use an interface any longer

For support of transactions via annotations Spring uses proxy classes. Whenver you make a call inside the same class the @Transactional annotation is ignored. We have to make sure that the deletion of existing corpora at import is running inside it's own transaction (otherwise the "facts" table is blocked after dropping the child table). Thus we have to use a different class with it's own proxy for this task.
Move checkAndRemoveTopLevelCorpus to DeleteCorpusDao and lock the repository_metadata table instead of the corpus table to mark a running import.

Since we want to delete rows from the corpus table from a different transaction we cant' use it as a lock.
removing DEBUG level for transaction management
don't query the facts parent table in GraphWithClauseGenerator but the correct child table
fix issues where you can't query subgraphs of the database when an import is running (only happend if the corpus was overwritten)