Adding mysql variable max_threads for TiFlash.

[mengxin9014]

Enhancement

Need adding mysql variable max_threads for TiFlash.

Now if we want change max_threads in TiFlash, we need modify TiFlash config and restart TiFlash.
So we add mysql variable max_threads that we can just set max_threads in TiDB client to make the param go into effect in a session immediately.

[JaySon-Huang]

... and restart TiFlash

actually, we don't need to restart TiFlash after changing max_threads in the config file. It will be automatically reload once the config file is changed.

tiflash/dbms/src/Server/Server.cpp

Lines 1250 to 1253 in e845908

	global_context->setConfigReloadCallback([&]() {
	main_config_reloader->reload();
	users_config_reloader->reload();
	});

[mengxin9014]

... and restart TiFlash

actually, we don't need to restart TiFlash after changing max_threads in the config file. It will be automatically reload once the config file is changed.

tiflash/dbms/src/Server/Server.cpp

Lines 1250 to 1253 in e845908

	global_context->setConfigReloadCallback([&]() {
	main_config_reloader->reload();
	users_config_reloader->reload();
	});

Maybe it needs to update the config file on each node.
Detailed requirement is here： https://pingcap.feishu.cn/wiki/wikcnHqT9XlW24wHVLRwwyHMLCh#1JcZA7

[JaySon-Huang]

Related TiDB issue: pingcap/tidb#34298