[C++] Introduce userver

[itrofimow]

This PR aims to incorporate https://github.com/userver-framework/userver into benchmark suite

[itrofimow]

@apolukhin @userver-framework can you guys please have a look at this?

There def. are some configurations tweaks to make, but i'm not yet certain which ones exactly - guess it can be done after merging this PR and seeing benchmark results.

[apolukhin]

A few nitpicks for later changes:

• Please write userver in lower case
• You could replace userver::formats::json::ValueBuilder with formats::json::StringBuilder to slightly speedup the service
• Adjusting ev and worker threads count would improve the results. Experiment qith those valies to find the best ones.
• If target architecture is know, use -march= and -mtune= compiler flags to save a few more CPU cycles

[itrofimow]

A few nitpicks for later changes:

• Please write userver in lower case
• You could replace userver::formats::json::ValueBuilder with formats::json::StringBuilder to slightly speedup the service
• Adjusting ev and worker threads count would improve the results. Experiment qith those valies to find the best ones.
• If target architecture is know, use -march= and -mtune= compiler flags to save a few more CPU cycles

Fixed userver spelling, added march=native.

• About formats::json::StringBuilder - quoting TFB docs, "All test implementations should be production-grade", and i'm not sure how production-grade that is (but feel free to disagree and fix it later).

• Adjusting ev and worker threads count: that definitely should be done, but i feel like it's best changed with some results at hand

[itrofimow]

@nbrady-techempower Hi! Could you please have a look at this?

[apolukhin]

About formats::json::StringBuilder - quoting TFB docs, "All test implementations should be production-grade", and i'm not sure how production-grade that is (but feel free to disagree and fix it later).

We use it quite a lot in production for serializing responses. It's an interface for streaming serializer (SAX), which is a quite a common technique. There's also a separate section in docs with a sample for SAX streaming https://userver.tech/db/dd7/md_en_userver_formats.html#streamingserialization . However, it may not provide notable advantage for small responses.

[nUl1]

an inline builder (formats::json::MakeDoc) should perform better here

[itrofimow]

Not sure what went wrong here, runs fine in my action queue: itrofimow#1

@nbrady-techempower could you please rerun failing checks for me?

[itrofimow]

Hi @nbrady-techempower!
Thank you for rerunning the checks.

Is there anything holding this PR back from being merged?