domain information schema may be wrong if switching schema cache v1/v2 and getting snapshot information schema

[wjhuang2016]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

When switching schema cache v1/v2, we would clear the whole schema cache.

	if isV1V2Switch && schemaTs > 0 {
		// Reset the whole info cache to avoid co-existing of both v1 and v2, causing the memory usage doubled.
		fn := do.infoCache.Upsert(is, schemaTs)
		do.deferFn.add(fn, time.Now().Add(10*time.Minute))
	} else {
		do.infoCache.Insert(is, schemaTs)
	}

After getting the snapshot schema, the only schema in the cache is the snapshot schema, which is not the newest schema.
Then domain.Infoschma is wrong.

// InfoSchema gets the latest information schema from domain.
func (do *Domain) InfoSchema() infoschema.InfoSchema {
	return do.infoCache.GetLatest()
}

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

N/A

3. What did you see instead (Required)

N/A

4. What is your TiDB version? (Required)

master