feat: support to modify configs without restart

[levy5307]

Support to modify configs without restart.

NOTE

1. String modifications is not supported.

2. A example for http interface to update config:

http://localhost:34801/updateConfig?max_concurrent_bulk_load_downloading_count=5

3. Please add DSN_TAG_VARIABLE(name, FT_MUTABLE) if you want to make a config mutable.

4. I will add validator for config update, in next pull request.

[hycdong]

I review this pull request roughly, I think this pr implement a http get interface to support user update some configurations dynamically, we don't need to restart server any more. I recommend that you add some comments in code or descriptions, including:

• The example of how to update the flag, the sample request and response
• The restriction of this pr, such as string config is not supported, the name of configuration should be FLAGS_##name, old configuration is not supported etc.

By the way, would you consider adding another interface for listing names of the config who can be updated through http?

[levy5307]

I review this pull request roughly, I think this pr implement a http get interface to support user update some configurations dynamically, we don't need to restart server any more. I recommend that you add some comments in code or descriptions, including:

• The example of how to update the flag, the sample request and response
• The restriction of this pr, such as string config is not supported, the name of configuration should be FLAGS_##name, old configuration is not supported etc.

By the way, would you consider adding another interface for listing names of the config who can be updated through http?

Yes, I will add a http interface to list names of all of the configs in other pull requests.