Goroutine leak in transaction retry

[djshow832]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

Connection 1:

create table test(id int);
split table test between (0) and (2000) regions 2;
insert into test values(1), (1001);
set session tidb_distsql_scan_concurrency=1;
set session tidb_disable_txn_auto_retry=0;
begin;
update test set id=id+1;

Connection 2 (cause connection 1 to write conflict):

begin;
update test set id=id+1;
commit;

Connection 1 (continue the transaction):

(select * from test) union (select * from test);
commit;

2. What did you expect to see?
   Check http://127.0.0.1:10080/debug/pprof/goroutine?debug=1 and see no goroutine leak.

3. What did you see instead?
   Check http://127.0.0.1:10080/debug/pprof/goroutine?debug=1 and see goroutine leak.

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?
   v3.0.7, master, etc.

5. Reason
   The reason is that (select * from test) union (select * from test) is not treated as read-only statement, so it's executed again when retrying the transaction. But the result set is never closed because the statement is not expected to produce results.