Expose options['result_buffering'] to CRM_Core_DAO

We have a custom contact search form task. The search parameters could easily result in more than 200k contacts being returned to this task, which often consumes PHP's memory_limit. Examination of the code shows that DB_mysql->simpleQuery() has the ability to use mysql_unbuffered_query() instead of mysql_query(), with the actual behavior determined by the setting of DB_mysql->options['result_buffering']. DB_common (and thus DB_mysql) has the setOption() method to allow for setting this option, but it is not utilized by CRM_Core_DAO. Given that CRM_Core_DAO is the fundamental object used for database interaction within Civi, I'd like to suggest allowing for manipulating DB_mysql->options from within the DAO object.

1) create a setOptions() method for CRM_Core_DAO to allow for setting options within the DB_mysql object. The proper object to be referenced is found in the global $_DB_DATAOBJECT['CONNECTIONS'], indexed by CRM_Core_DAO->_database_dsn_md5.

2) Modify CRM_Core_DAO->executeQuery() to recognize a new parameter. The new parameter should be an array("option_name"=>"option_value"), corresponding to the options available in DB_mysql->options. If the new parameter is populated, executeQuery() should call $dao->setOptions() prior to calling $dao->query().

We like to get core team's thoughts on this change before implementing it in our own code.