fix(query): max_threads can not determined automatically

[TCeason]

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Summary

Fix max_threads can not determined automatically

The max_threads has default user_settings user_setting: UserSetting::create("max_threads", UserSettingValue::UInt64(16)),

If a connection connect to databend , it will set max_threads = 16, then this if branch will not cover.

// Overwrite settings from conf or global set.
        {
            // Set max threads.
            if ret.get_max_threads()? == 0 {
                let cpus = if conf.query.num_cpus == 0 {
                    num_cpus::get() as u64
                } else {
                    conf.query.num_cpus
                };
                ret.set_max_threads(cpus)?;
            }
        }

Test on local

➜  ~ cat /proc/cpuinfo|grep process|wc -l
24
➜  ~ mysql -uroot -P3307 -h127.0.0.1 -e "show settings like 'max_threads'"
+-------------+-------+---------+---------+--------------------------------------------------------------------------------------------------------------------+--------+
| name        | value | default | level   | description                                                                                                        | type   |
+-------------+-------+---------+---------+--------------------------------------------------------------------------------------------------------------------+--------+
| max_threads | 24    | 0       | SESSION | The maximum number of threads to execute the request. By default the value is 0 it means determined automatically. | UInt64 |
+-------------+-------+---------+---------+--------------------------------------------------------------------------------------------------------------------+--------+

Closes #8706

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1 Ignored Deployment

Name	Status	Preview	Updated
databend	⬜️ Ignored (Inspect)		Nov 9, 2022 at 5:48AM (UTC)

[TCeason]

cc @wubx

[TCeason]

Dev MacOS / test_stateless_cluster_macos(optional) (pull_request) Failing after 14m

Failed. other test is ok.