planner: session memory usage cannot be released after disconnecting

[qw4990]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create a new session and run the code snippet below:

	mustExec(c.ExecContext(context.Background(), "set @@tidb_opt_fix_control='44823:0'"))
	mustExec(c.ExecContext(context.Background(), "create table if not exists t1 (a0 int, a1 int, a2 int, a3 int, a4 int)"))

	for i := 1; i <= 1000; i++ {
		tmp := "(?,?,?,?,?)"
		values := strings.Repeat(tmp+",", i-1) + tmp
		st, err := c.PrepareContext(context.Background(), "insert into t1 values "+values)
		must(err)

		var vals []any
		for k := 0; k < i*5; k++ {
			vals = append(vals, 1)
		}
		_, err = st.ExecContext(context.Background(), vals...)
		must(err)
	}

Then disconnect this session, trigger GO GC, take a look at the memory usage, and find the session's memory is not released:

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

TiDB after #48152

5. Analysis

It seems like this PR put every connection handle into a global list, which makes the GO GC cannot release these connections: