Change storage engine of log tables

mysql.slow_log and mysql.general_log are CSV engine.

CSV engine is hard to make secondary indexes.

So change storage engine of log tables and make secondary indexes.

Sample : mysql.slow_log

SET GLOBAL slow_query_log = OFF;
ALTER TABLE mysql.slow_log ENGINE = MyISAM;
ALTER TABLE mysql.slow_log ADD INDEX idx_start_time (start_time);
SET GLOBAL slow_query_log = ON;