Improve fiber gc registration

[kyswtnb]

Hello!!

@matsumotory Please review.

When repeatedly performing Nginx::Async.sleep, it overflows the value of MRB_ARY_LENGTH_MAX, resulting in the following error.

https://github.com/matsumotory/ngx_mruby/blob/af4228888587680d2c44b0aebbfb91e6e470d219/mruby/src/array.c#L26-L29

The issue also occurs with ngx_mruby v2.5.0.

There might be a misunderstanding on my part, or something missing. Please point out any discrepancies or omissions.

After looking into it, it appears that the error is occurring because mrb_gc_register is executed on every sleep operation. If mrb_gc_register is run at the start of the fiber and mrb_gc_unregister is run at the end of the fiber, the error will no longer occur.

I tried creating a PR. Please check it.

Pull-Request Check List

• [ ] Add patches into src/.
• [ ] Add test into test/. Please see about test docs.
• [ ] Add docs into docs/ if you change the features such as build system, Ruby methods, class and nginx directives.

[matsumotory]

Thanks @kyswtnb !