Changing WAL mode requires obtaining an exclusive lock on the database and can only be done when there are NO other active database connections. Check that this is really the case, and bail with a useful error message if an application attempts to change WAL mode while transactions are in progress. Expose disableWriteAheadLogging() in the API. Change-Id: I87599de3b88c53dcd75677aefd72e40de216c2c1