try use max uint64 as tso for table/index reader which only has 1 cop task and in autocommit txn

## Enhancement

The point-get query can use max uint64 as tso when only has 1 rpc request.

Similarly, try to use max uint64 as tso for table/index reader which only has 1 cop task, and in autocommit transaction.

Here is an example:

```sql
create table t (id int key, b int, c int, index idx(b));
```

For the following query, only read a small range of data, and in most cases, there will be only 1 cop RPC request, then we can use max uint64 as tso too, there will be some performance improvements for small queries.

```sql
select * from t where id > 1 and id < 10;
```

### Drawback

This change may break linearizability, so this optimization can only be used when the `tidb_guarantee_linearizability` is disabled.