fix(api): dynamically sets log level for timers pre-created by timer-ng

[gruceo]

If the log level was dynamically set, we must also change the log level for all timers pre-created by the timer-ng library.

We're going to use the global _G table and not a shared dictionary due to performance reasons.

[ADD-SP]

Just a note, we made a workaround within timer-ng, and while it still has some limitations, we're at least going to make this feature (dynamic log_level) work.

In the future, we should rewrite the C code to implement this feature elegantly, and @gruceo will create internal tickets to track this thiing.

[locao]

@gruceo can you check the failed tests, please?

[dndx]

@gruceo Could you put the ticket number for improving dynamic log level in a comment? We need to properly address the hacks we introduced inside our codebase through proper C code change and @ADD-SP can help with that.

[dndx]

I suggest we take this PR out of 3.2.0 as the hack is too specific to the timer library we use and I have a feeling that there is a better and more systematic way to fix these and similar issues. @ADD-SP can help get this moving towards that direction.

@kikito @guanlan any objections? This does not looks too urgent to me.

[gruceo]

@dndx thank you for your suggestion, I dropped a comment to KAG-457. Tests are passing now. Note this bug does not happen with native ngx.timer API. The alternative way to make dynamic log levels work with our timer-ng library is a complete rewrite of our dynamic log levels C code, and it may require a patch (see KAG-457 for details). I suggest we ship this PR as-is for now until we complete work on KAG-457 and since we already uploaded to LuaRocks a new version of the timer-ng library with this fix I don't think it makes sense to merge #10193 without this PR. What do you think? (cc @fffonion @ADD-SP)