Fixed exceptions on attempts to serialize DatabaseSqlite. Encountered today by Nike...

Fixed exceptions on attempts to serialize DatabaseSqlite. Encountered today by Nike, earlier seen during unit tests. Not implementing __wakeup() because restoring a connection along with its full state would just cause obscure bugs. If null mConn will cause problems (unlikely, because most instances of db object serialization are accidental and the connection is not supposed to be used after deserialization), we'll tweak specific classes that store database objects.