Allow different transaction isolation levels in db and some misc changes

src/api/database/__init__.py

@@ -5,6 +5,7 @@ from api.database.database import (PreparedStatement, FilledStatement,
                                    AbstractTransaction, ReadTransaction, WriteTransaction,
                                    DbConnection,
                                    DbConnectionPool,
+                                   TransactionIsolationLevel,
                                    DbResult, DbResultSet, DbResultRow, DbValueType, DatabaseWarning,
                                    DB_RESULT_EXCEPTION, DB_RESULT_WARNINGS, DB_RESULT_SET, DB_RESULT_AFFECTED_ROWS)