bug: session leak in execute explain pipeline

[sundy-li]

Summary

It could be reproduced in cluster mode (>=2 nodes)

create table t1(a integer, b integer, c integer, d integer, e integer);

EXPLAIN PIPELINE SELECT (SELECT COUNT(*) FROM t1 AS x WHERE x.b < t1.b) AS col1 FROM t1 WHERE a > b - 2 AND a < b + 2 OR c > d ORDER BY col1;

show processlist;  -- the explain query still exists