Allow ProxyServer to run only one server at once.
Insert dummy AntiThrottler class
Add ManuallyControllerMockServer
Make AntiThrottler prevent exceeding total simultaneous request count
Rename auxilliary spec classes
Make MockServer expose latest requests' paths
Make HttpMessageReader extract query params
Add InspectableMockServer#dont_hold_requests
Extract `make_async_request` to spec helper
Close session after each 5 API2Cart requests
Extract guarding functionality to TotalSessionCountGuard
Play API2Cart's rules in case of many *simultaneous* requests