Update of the TechEmpower Benchmark code

Looking at the latest TechEmpower Banchmark 21 of 2022 I saw that Drogon is only ranked 69 for the JSON serialization test. It's using a very old unmaintained JSON lib and maybe just in general the code could be improved to perform better in the benchmark? The code there is 4 years old. Would be sweet.

Repo: https://github.com/TechEmpower/FrameworkBenchmarks/tree/master/frameworks/C%2B%2B/drogon
Benchmark: https://www.techempower.com/benchmarks/#section=data-r21&test=json