Fix DBs

Several fixes on DB classes(AccountingDB, SystemLoggingDB, UserProfileDB, TransformationDB, JobDB, PilotAgentsDB) to:
- avoid the use of connection.close()
- avoid the use of getConnection()
- avoid the use of _insertFields
- add some further test cases.

This comes from a review after the new movement to the new MySQL implementation with a persistent connection per running thread.