Improve serialization speeds of JsonOutput while maintaining backward compatibility

I'm going to rewrite the JsonOutput implementation without changing the public API. I'll try to take into account the Boon experience and others Json libraries. Also, I'll try to take into account all feedback around benchmarks (discussions on the mailing list):
https://github.com/RichardHightower/json-ser-test
https://github.com/bura/json-ser-test