Bug 35624

Before running any transaction step on the DB thread or the main thread, we check if the database is closed by calling the SQLTransaction::checkAndHandleClosedDatabase() method. Parts of that method though are intended to run only on the DB thread, and could cause a crash if run on the main thread. In particular, calling SQLTransactionCoordinator::releaseLock() on the main thread after SQLTransactionCoordinator::shutdown() was called on the DB thread triggers an assertion failure. Solution: check what thread we're on inside SQLTransaction::checkAndHandleClosedDatabase() and only run the appropriate code for that thread.