Don't use strftime()
Some optimization

- don't copy pointer for readability
- remove debug output
- for now, only use :path header
- optimize check_path
- don't set 200 status code per request and implement mrb_http2_200_send_response
Use fstat() instead of stat()
Cleanup code
Merge pull request #8 from matsumoto-r/some_optimize

[WIP] Some optimization