Add tracing capabilities

[php-coder]

In order to improve logging and simplify debugging, let's pass random number that uniquely identifies every request.

• pass $request_id in the Request-ID header to a backend
• include request id to the logs on nginx side
• include request id to the logs on applcation side

Resources:

• https://www.nginx.com/blog/application-tracing-nginx-plus/
• https://gongled.ru/blog/2017/01/05/tracing-requests-in-nginx.html (in Russian)

[php-coder]

BTW here is the similar (?) for HAproxy: https://cbonte.github.io/haproxy-dconv/1.8/configuration.html#4.2-unique-id-header