Feature: support SET_VAR hints

[sundy-li]

Summary

Description for this feature.

https://dev.mysql.com/doc/refman/8.0/en/optimizer-hints.html

SELECT /*+ SET_VAR(sort_buffer_size = 16M) */ name FROM people ORDER BY name;
INSERT /*+ SET_VAR(foreign_key_checks=OFF) */ INTO t2 VALUES(2);
SELECT /*+ SET_VAR(optimizer_switch = 'mrr_cost_based=off') */ 1;

[wubx]

Processing some simple SQL requires more concurrency:

select /*+ set_var(max_threads=1) */ name FROM people ORDER BY name;