RFC: Request Shadowing

[AndrewDryga]

Redirecting or shadowing (duplicating) small percent of requests to a staging environment works really well for complex projects with microservice architecture. In some cases, shadowing allows getting rid off "monolith-style" integration tests entirely (when people trying to test at the same time).

This plugin can be fairly simple to implement: with given probability we duplicate request to a shadow back-end and then persist diff between responses in a database.