The dump stats and DDL event handling may conflict

[Rustin170506]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

func TestDumpStatsDeltaBeforeHandleDDLEvent(t *testing.T) {
	store, dom := testkit.CreateMockStoreAndDomain(t)
	tk := testkit.NewTestKit(t, store)
	tk.MustExec("use test")
	tk.MustExec("create table t (c1 int)")
	// Insert some data.
	tk.MustExec("insert into t values (1), (2), (3)")
	h := dom.StatsHandle()
	require.NoError(t, h.DumpStatsDeltaToKV(true))
	// Find the DDL event.
	event := findEvent(h.DDLEventCh(), model.ActionCreateTable)
	err := statstestutil.HandleDDLEventWithTxn(h, event)
	require.NoError(t, err)
}

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

It works well and we can tolerate the error.

3. What did you see instead (Required)

It bails out an error.

4. What is your TiDB version? (Required)

master